Ok, so, further into Expert mode.....
.....and I think it needs to get harder yet
The only Exo that ever gets blown up is my Siege guy, and that's only if I'm not paying enough attention when I fire the rockets; usually when I dont realize that I'm standing near some of those thunder runes or whatever they're called. Other than that, I'm tearing through each mission pretty quickly. Avoiding damage entirely is a good bit tougher, and there's more "threatening" bots than before, such as the damage/defense/movement buffing bots, but there could be more threat.
One definite thing: The siege bot's rockets are stupidly powerful. So very much damage, affecting so very many tiles. Every splash damage increase making it even more absurd. Some missions can be beaten with only a few of these.
Also, as pointed out here:
http://www.arcengames.com/mantisbt/view.php?id=12416 I dont think Expert mode is increasing enemy levels quite right. Levels though, for the enemies, honestly I dont think they do enough. Many of them dont increase in threat all that much at all when they level up, which also has the side effect of making the factory missions seem a little pointless. It seems like any given enemy bot needs ALOT of levelups to become distinctly more dangerous, and of course during that time my team is increasing in power faster than the bots are.
Player stats though, are feeling quite well balanced, even if some base weapon numbers (like the rockets) could do with some tweaking. The various items seem nicely balanced, though there's a couple of types that seem underwhelming, but I'll wait and see how they are in the late game in this mode before suggesting anything.
And a suggestion: Increase the "awareness" range of enemy bots in general with each difficulty increase. Simply increasing the number of bots per level only does so much when only a certain amount of them can ever come at you at once.