The one potential exception here is Pia Nalaar, which, while she does not technically have improvise, does provide a healthy set of stats and a relevant ability
As such, you can cast one side first, and later unlock the other side (the other door)
While 8 mana is certainly a lot, and that does preclude it from being in many Commander decks, basically any Green Commander deck should be playing Ugin, since its so easy to produce the mana in that color
The collection manager is mature and full-featured
The Ruinous Powers deck is Blue, Black and Red, and wants you to sacrifice your opponents for additional strength and play demons
Its played in over 50% of eligible EDHREC decks, which shows how popular of a removal spell it is