If I loved Ultimate Alliance should I play it?
I'm going to go with yes. I never played Ultimate Alliance but have just read some summaries and watched a couple of minutes of cut scenes and gameplay and there are both big similarities and big differences. For the similarities, in both games you build a roster of known heroes who are voiced individually and have their own lines and dialogue and select from your roster for missions (it looks like). Some story missions in Midnight Suns have particular character requirements and some are generic ones. Mostly you have free pick of who to take and can focus on improving those heroes the most if you wish. I didn't notice that UA has generic missions. In Midnight Suns, doing some generic missions is a good way to build up resources and levels before taking on a story mission. I saw cut scenes in UA and Midnight Suns has those but it also has a lot of free roaming interaction where you can walk around the team's base and have conversations and do activities with the heroes. (Pro-tip, don't try to take Wolverine landscape painting). I don't know if UA has that level of out-of-mission scope. Midnight Suns has three distinct game components - missions, social interaction and exploring / (light) puzzle solving. From what I saw, UA is focused on missions. Though I could be wrong. In Midnight Suns you can pretty much ignore the exploring and social interaction if you want, though both are ways to build up your resources and power. Heroes work together better if you've built up friendships and the team spirit.
So other big difference is the style of combat. UA looks like roaming real-time team with some nice nuanced combos and strategies. Looks fun. Midnight Suns is turn-based and takes place in fixed arena-style locations. That immediately makes it sound duller than UA but it's really pretty deep. Much more so than it or its card-based look and feel would suggest. The only randomness in Midnight Suns is the initial shuffle of the deck. (I think their use of visual metaphors of cards and decks is one of the things that put a lot of people off because people thought of it would be a collectible card game style thing, I think). Everything past the shuffle is deterministic. So if a power says it will do 32 points of damage and stun an enemy, that's what it does. Sounds like a weakness but it's actually one of the best features because you end up trying to work out a precise sequence of plays and movements that will lead to victory. You're a little like Stephen Strange searching futures trying to find one way that will beat Thanos.
So it depends what you liked about UA. Marvel character's accurately portrayed and well-voiced? Yes, you'll love it, it has this in spades. Point and clicky real time combat? It doesn't have this so it depends if you like playing Death Sudoku with an array of powers and play styles. All the heroes are quite distinct which is impressive (but then this is by the team who did XCom). Even when you're comparing heroes that seem to play similar rolls, e.g. Cap and Wolverine are both very resilient team members who can somewhat soak up damage and make your team more tanky. But Cap is way better at protecting other team members with taunts, granting block whereas as Wolverine is more of a light attacker who wont stay down (my Go To for bringing down minions in bulk). If you don't like lots of free-roaming exploring and uncovering the story, you might not like it as much. Though you can skip out on that and still get the essentials of the story.
If you play it let me know what you think. If you're starving for decent Marvel content which there's a dearth of these days, it'll certainly give you lots of time with some of the best Marvel characters. Unlike UA (from what I could gather), you have an original character you play as in Midnight Suns, so you are actually represented in the game and interacting with the heroes. But your character is very well integrated into the lore and background.