Ugin is an un-fun design cannibal
Ugin, the Spirit Dragon is a design cannibal. Arena would be a more diverse and enjoyable game without him.
Playing with him:
Ugin undermines the design choices of the decks that use him. There are lots of very interesting top-of-curve spells that could be used in long-game decks. When Ugin was reprinted, the hard work and creativity that went into designing those cards was stamped out by one obvious 'best' solution. Ugin (and to a less offensive extent, Ulamog) are the best top end for virtually any long-game deck. None of these decks need Ugin, but all of them are best served by using him. So they are pressured to ignore the rest of wizards' design space in that area.
Playing against him:
Obviously, this is miserable. How many players see their opponent resolve an Ugin and think "wow this game is really clever"? The feeling is more like "Wow, my opponent has just exiled all personality from the game. Why am I playing this?" Worse yet, the feeling begins for many players as soon as they see that their opponent is likely to have Ugin in their deck. I'm not against sweepers. They're essential. But usually there's some way to play around them. Some way to diversify your threats. And Ugin isn't just a sweeper, he's a single-card inevitability powerhouse that can also one-sidedly sweep the board in a way that's uniquely hard to prevent. It's not fun to see, or to see coming.
"But there's always a 'best' card! Ban or suspend Ugin, and ramp players will just play something else that's only slightly less busted as their finisher!" Sure, that's true. But I think Ugin is an extreme example. His inevitability is unmatched by any other card. And since he's colorless, he competes against every other card. There's nothing else waiting in the wings that any deck could play, and that does everything he does, but just a little worse. Remove him, and deck designs can't just replace him with 'Ugin lite'. He would be replaced by a variety of cards that would need to be carefully chosen to fit the deck they go in. That diversity would make magic better.
Sorry for the rant. I like long games, but I don't want to play with Ugin or against Ugin. He invalidates a lot more than he adds.