Personally, I think trained units can be upgraded just fine, other than group size and trinket equips. Giving them attack and defense bonuses on top of their equipment upgrades is a bit too much, IMO.
I agree with this. I think that the current level-up bonus system (just accuracy and health improvements) is perfect for trained units and dragons (which have HUGE attack and defense stats already!).
For non-customizable, "ungroupable" units (ie. summons, wildlings, trolls, etc) an increase in other stats is necessary to help them scale.