I haven't used all the exos yet, so I can only talk about the four I have. But of those, here's what I think of each one:
1. Assault.
Early game I used this guy all the time. The laser rifle has good damage and range, the grenade launcher is handy, it's tough, and if you get into ammo trouble the machine gun is backup. Was my workhorse for a long time, and I think it's an overall great bot in the first half of the game. I upgraded the laser rifle with range, ammo, and damage, and had a field day. Even more fun once I got some sentry turrets on him. Three shield slots starting out and the shield stat bonus inherent to the exo also mean he's more likely to survive mistakes or simple suboptimial situations ("I'm out of stealth, turrets, there's no cover, and there's just 5 bots left to kill. CHARGE!!!"). You have fewer resources in the early game, so that toughness is very handy as a fallback.
2. Siege
First half of the game, used sparingly. As in: "big clump of enemies, use rocket launcher". Almost never used otherwise unless I was in dire ammo trouble on my other bots. Low stealth (and only one propulsion slot) makes getaways dangerous if you do get into trouble, so it's a bot that I played very cautiously with and used sparingly.
3. Ninja
The welding laser is high damage for early game and can take out stuff in one hit that other weapons can't, but the range sucks. Has decent stealth, but still risky to use it a lot without upgrades. Not a lot of room for viruses. Also used sparingly, namely when I could stealth into stuff and get it close, or for avoiding traps (which you can do REALLY easily).
4. Science
This is probably the one I'd try to swap out if I could, simply because as a combatant it's... well okay, the lase rifle makes it not bad if you can keep range on your targets. If you get into range, it's super squishy. Limited upgrade space that isn't computers, but it's great at hacking doors. I'm not sure I could give it up easily given the limited slots for hacking on other bots, but you'd get a lot of firepower trading it out for something else.
Things changed with the epic bots though:
1. Epic Ninja
I used this guy a ton. By this point you can get a huge amount of stealth. The kinetic burst weapon can flatten most stuff. If you can't (Tigerbot, Doombot, etc), it's got enough computer slots to load up on viruses (and the stealth to use those viruses and get away safely). Excels at both assassination and reprogramming, and those are two very effective strategies.
2. Epic Siege
Ignoring the fact that the
plasma cannon is currently so absurdly overpowered that
I made a video of it one shotting entire maps, the rest of this guy is awesome. You still get the rocket launcher. You get massive range, wall ignoring shadow torpedoes for knocking dangerous targets off with impunity. You get extra slots to actually get a usable amount of stealth. You also get a third shield slot, for increased toughness. There's really a lot to love here.
3. Epic Assault
To be honest, this guy disappointed me. The plasma rifle is nice, but low ammo. The volatizer has a very weird firing pattern. He could really use a fourth reactor slot given that he has five weapons to power (especially if the -to power use in a system stat does wind up going away). The laser rifle is still good, but I found it trailed off in power as the game went on compared to what the Siege and Ninja were bringing to the table.
It's not that it's a bad exo and it's still one you can use a lot, it's just kind of a meh upgrade compared to the improvements the others got. Instead of being my primary bot, it was my "don't have a reason to risk one of my specialists right now" bot.
4. Epic Science
This was the only bot I really tried to get hacking points on, so I still didn't use it much to protect my hacking points from accidents. That said, the chain gun is awesome. Good range, lots of ammo, reasonable damage, and rapid fire brings the damage up to quite good if you set yourself up to use it. Three propulsion slots is nice, has so many computers that you can load up on all kinds of stuff (I had massive sensor range), and actually did surprisingly well when I was forced to use it in direct combat. The only one I had with only two shield slots, but the chain gun helped alleviate that as a risk. Probably different on hard where enemy ranges are going to be longer.
Of these four, I thought they were pretty well balanced. The epic assault was less impressive than the others, but still pretty useful. So... I don't know if this feedback is of any use or not.