This is a great concept. One thing I really wish it had - since when changing your squads items you can see what the upgrade does (like +1 speed, etc); this is not shown in the upgrade tree. I would like to see what the upgrade does compared to others to better weigh which one to go with. The visual upgrades on the items was a nice touch.
Also one thing that stood out to me, since you have the background repeating, it was nice to see that it wasn't noticeable since the distance you traveled to the next mutant pack was less than the repeat, so you were always stopping in a different spot on the tiles. That was done well.