I recently lost a high leveled party (1 level 6, 3 level 5 heroes) at a dungeon run. The first one was maybe not avoidable. I say maybe because I don't quite remember. I think he had bleeding, I had bandage, I decided I don't need it. Oh, it was also a Hellion, so I could simply use her ability to purge her. despite that I didn't do it, so she died. I think to some point it was of course the RNG screwing me over but of course tghis was also, because I LET it screw me over.
I decided to press on the quest with m remaining 3 heroes which was a very bad idea because all I had left was a vestal, a jester and a crusader. Not really a powerful group but I thought "you can avoid most battles because you have good scouting abilities". Went all good but I couldn't beat the quest from just that, so I walked in the first boss room. It went just as excepted, I was murdered awefully. If I wouldn't have pressed on, I had ony lost one member and at least got the gold and heirlooms from that point. But I was too stubborn to get that quest and lost. The game works pretty much that way. If you press on despite all being lost, the game kicks you in the ass. If you wait too long to retreat, the game kicks you in the ass.
About boss fights: Just tell it terrible design choice then. The only boss fight I lost heroes so far, is the Siren. Oh, I beat them all btw. And I didn't use the wiki to check what they do, not even the Prophet (that was remarable easy as I later found out). I Do check out the wiki occasionally but that is because I cannot remember all or because I want to see how much damage an enemy does on a specific difficuilty. Also, for some curios because I cannot find out how they work or because I forgot how it worked. The game is 80% memoraziation and 20% planning. Simply that. Sounds terrible maybe it IS terrible, but I like it that way.
So back to boss fights and regions. The brigand pounder is... well, the first time I met it and I let the shot happen I was shocked. Didn't kill me though, but I don't remember the details. The thing can also miss fire, so yeah, even if you screw up and cannot kill the matchman, the RNG might screw over the punder and it does 0 damage. Also, about group setup. I don't even know why you don't have something for the back row or why only one person. pPretty much any party should have heroes that can target anything because if you have a single turn they cannot hit something, they waste their potential, worse, they get stress because they simply had to pass the turn. Of course you cannot hit with a crusader the back row but at least you can heal or buff other heroes with him. Bulwark of Hope is awesome in boss fights because it increases his durability and draws the attention towards him for one turn. Doesn't work of course against the pounder because ti hits everythign but it could let him survive the blast (for an example), giving you tiem to retreat and come wit another group again. I said it before, I say it again, the first time you meet a boss, you notice you have the wrong party, retreat. I don't see the problem here. Is it a problem of the mentality? Have you a fixed mindeset that let's you believe you have to press on, no matter what, otherwise that expedition is failed? Look at my failed expedition at top. I did exactly that and lost more than I should. Sometimes you simply cannot rescue a single hero. Sometimes it's really the RNg that screws you over. But that's ONE hero right there. A good hero, yes, but that's not a terrible loss, I can always level another one as replacement. Now I have to replace 4 excellent heroes which is a bigger problem. It throws me back because I cannot do expert dungeons anymore until I have more level 5 heroes again. Boss fights work the same, if you are too stubborn and keep fighting despite noticing how ineffective it is, you will loose everything. ONE hero is not a terrible loss. I call this "expectant loss". It's soemthign I have to except to hapopen at very hard runs, e.g. boss runs.
I had to retreat many boss fights, especially at the start of my Darkest Dungeon runs when I was still new to the game, because I took the wrong party member with me. Now, when I plan a party, I always consider "Okay, this boss fight does this and this and is weak to this and this, so I take at least this and this hero with me.".
About the cove, I think I agree on this. However, you are exaggerate a little here. Phelaic guardian does no way that much damage to a single unti unless buffed. I just checked to be sure because I know he does rather low damage compared to the rest of the creatures there. His expert variation does at max 11 damage. At that point you have around 50 hp with most heroes, 60 with tanks (for heroes that take the beating from him).
So I think you either misunderstood something or you are lying. Or simply exaggerating to make your point. But you probably just didn't see or didn't tell that he was buffed by the shaman, because that's the only reason why he would do so much damage.
Okay, so after I put your technical mistake aside, I accept that the cove is pretty much fucked up. Remember that I said I almost never loose team mebers to any boss? Well, I also said that the Siren is the only one that kills team mebers. But to be more specific, she does this onb EVERY run. I don't know if I do something wrong with her (didn't check the wiki) but I think she is the most fucked up boss here. She has a full party AoE bleeding skill, full party stress skill (but then again, whcih boss doesn't have that?), a summoning skill (and since the monsters at cove are already strong on their own, the boss fights gets quickly out of control) AND can take one of yor heroes on her side. The only good way to deal with her is to have party members with high debuff resist, so she never gets you party members. Oh,a lso she has two actions per round because other stuff wouldn't be fucked up already. I have no problem with the witch having two actions because she is pathetic but the siren can turn your team mebers over and screw you with this AND inflict serious damage (either stress or status effects) on your team AND can summon reinforcments. I may say that every other boss is simple to be fight with normal means (mostly) but this fights is ridiculous. Is however the only one though. And yes, I agree that the cove is arguably harder than the rest. I first thought this about the warrens until I noticed, besides a lot of diseases they don't do that much of a threat.
The cove has strong attackers, the pelagic grouper does over 10 damage and they always coem in big groups (as the name implies). The jellyfishes have a lot of strong debuffs to screw you over. The guardian is pretty much the tankiest enemy in the entire game, more so if he protects another enemy. Also, fr some reason the increased protection statys even after he dropped to guard his team member. Even with DoT effects it takes long to wear him down and killign the other members first won't do much either because he simply protects them.
The shaman can stress two heroes at once and while the stress is not over 20 (unless your heroes have increased stress intake which... was your fault then), it still is over 10 which means he only has to do this ten times to screw you. However, the cove is an excellent spot to purge negatives quirks from your heroes. The cove has (wuite frequently) a coral that, if you use medical herbs, purges to 100% a negative quirk. And unlike the pergament scroll of the warrens, the coral appears almost in every dungeon. Twice. The thing is so freaking awesome, I always take enough herbs with me to use it. 1 negatove quirk less for just 200 gold is too awesome to pass. I don't say this is the reason or this should be the reason that the cove is harder. But I say, at least you get or can get awesome stuff if you travel through it. I think the cove has also the best ratio of benefical curios to damaging curios of any dungeon but I would have to check the wiki to make sure but in my experience, the cove has only a few curios that are always harmful. Hell, I can only remember one at this point and that is this big stoney altar thing or whatever it is.
About stress: I think you are right, long quests should be more stressful but aren't because of resting. It's an odd design choice, not one I would mind because otherwise on long quests your stress would skyrocket.
Heirlooms, yes, same problem at my current run right now. Actually, the only buildings that have that ridiculous requirements are the guild and the blacksmith and both are very much needed to progress. I think that heirlooms should stack more with antiquarians, just like jewels and gold does but oddly enough it doesn't. That's really terrible. I now switch beteween gold runs and heirloom runs frequently, so I either ignore all heirlooms or all gold on my way, so I have more space for the other. Since the introduction of the antiquarian, gold is no problem anyway since she can find on every curio a 500 piece that stakcs absurdely high. I have yet to find the stack limit of that one actually. She even finds this stuff on curios ith a fixed loot, for example the torch stand.
I plan my heirloom runs accordingly and look, what heirlooms I need. The long run where I wiped my party was btw because I wanted 18 manuscripts that awaited me for the win. I got greedy and wnted it so bad, that I risked (and lost) my entire party. I still could get out with at least soemthing, now I have nothing. I even lost very rare trinkets that I worked so hard for. I probably won*t make the same mistake twice at least not so soon. I'm sure I will screw up eventually, but I can laugh at my own misfortune. In the end, it was my own fault, so I cannot blame the game for that.