You sure? From what I gathered from the demos, they stack.
Terraria Community Forums
Kazzymodus
Jeckel
They did change it from 5, but not much. Now you can combine two units together, but you can't really stack them. I guess it would be more accurate to say it is now 2UPT instead of 1UPT. Still an overly simplistic system that I hate. I'll take stacks of doom over carpets of death any day of the week.
Your Conscience
You can actually combine up to 3 units, but they don't really have the combined power of 3 units. But in all honesty it's kinda petty not to buy a game due to a small mechanic like that. So far, I haven't had much experience with 6, but I certainly prefer Civ 5 over Civ 3. Who knows, maybe it's just me.
Jeckel
It isn't a small mechanic though, it is a major mechanic that drastically effects one of the major aspects of the game. A buddy preordered 6 and we've put over a hundred hours in so far and while the game has some great ideas, it all ends up being pretty shallow once you get farther into the game. Currently the biggest problem is the UI which seems like it was made by someone that failed GUI design 101.
Your Conscience
I can agree with you on the UI, it kinda feels overly simplistic. But personally, I really have no issues with the combat/troop distribution in 5 or 6. Maybe it's just me, but I like having a massive army that's spread out rather than condensed in just a few tiles.