TLDR: MU has been one my best gaming pick ups in a long time due to theme and flexible play group appeal.
I find Marvel United to be amazing. It fits well in that slot that has more going on than a party game, while also has the theme and pacing to hold the interest of non-gamers. It is a solid, highly modular cooperative resource puzzle in 30 minutes or less.
I see the “it’s for kids” thing thrown around a lot. I felt it before I really knew the game. In fact, that is why I skipped the first KS—and then deeply regretted it after trying the game. It cost me a bundle to chase it down later. I went all in for the MU X-Men though.
MU is very good, MU X-Men is better as they expanded in their design space (more unique cards per hero, starting hand cards, bottom deck cards, more interaction with timeline (cards already played), etc. Not that MU is obsolete. It all mixes so very well.
Yea the amount that is KS only is a crime. Especially when the game thrives only if you have enough to swap heroes, villains, and modes around a bit. I do think there is or will be enough at retail between MU and MU X-men to be way more than serviceable. I would not suggest just a core box. You need a critical mass of options for it all to feel great.
As to the timeline, I use the circular Neoprene mat from the KS. The cards just go in a circle. It does eat space but it’s nice to look at the full timeline as a thematic story presentation of turn and it also has game value as plenty of cards interact with the timeline. Also I tend to have 3 to 4 people at the table so the large circular layout works to equalize position.