Ok, so, I've spent some more time with Expert mode, going about 1/5th of the way through the game. So far, so good.
This is definitely a step up from the other modes; you need to be able to make good use of the special items like mines and sentries to be able to really make it here. Item power now raises a bit faster from one level to the next, so there's stronger things to get and my guys tend to have a good amount of shielding, and this works out nicely with the rate at which the enemy bot power is increasing. Some of these guys are pretty damn tough. Also, SilenceBots. BAH. That's what I think of them.
I've only had a couple of actual deaths from enemy attacks, and a few deaths that were my brawler dissolving himself (oops). But it's alot harder to completely avoid damage here, particularly when you get enemies like the TeslaBots that have 20 range, and Commands/Masterminds giving them loopy buffs.
It's easy to screw up in this mode, so you've really gotta be paying attention, and maxing out your stats as much as possible is a must. So that part is going well.
Some of this may change a bit with the Bahamut upgrades, but I havent found any of them yet, for once they didn't generate overly close to the starting point.
The one thing that has been kinda annoying has been the enemy bot selections; I know that in the early part of the game it is basically using a hand-picked bot selection to give the player a bit of time to power up before letting the RNG go nuts, but doing 6 missions in a row that's all CommandBots, Masterminds, SilenceBots, and some snipers and such, the same ones each time, can get old. My suggestion is to create multiple hand-picked groups... like 3 or 4.... that the game can randomly choose between for the early game missions. Simply to keep it from getting stale.
Overall, it's providing a good challenge. No problems yet, and quite enjoying it.