1UPT is pretty decent in theory... the problem with this system in CiV boils down to the following in my opinion:
- Singleplayer: The AI is thoroughly incompetent on the tactical field. On lower difficulty levels wars are simply a pushover because of this, on higher difficulties (immortal and deity mostly) the sheer amount of troops the AI can output makes for boring grinds trying to level up your units to get some super upgrades like +1 range or double shot... which the AI never ever gets.
- Multiplayer: Competitive MP wars are clickfests. There is a fair level of strategy and tactics involved in the positioning thanks to 1UPT, which is good, and an experienced player can easily sweep the floor with a rookie while being outnumbered 2 to 1 purely because of the tactical layer that 1UPT adds. But more often than not, wars are decided by who gets to move and shoot first at the beginning of each turn, which depends on non-game technical factors like connection and server. Simultaneous turns in a turn based game are simply not compatible 1UPT.
Anyway, going back to doomstacks would be a massive loss in my opinion. However flawed 1UPT may be both in SP and MP, doomstacks are worse. So I'm quite curious about the possible improvements that Civ VI could bring to this aspect of the game.