Why Couldn't the PlayStation X-Men vs. Street Fighter Do Real Tag Teams?
Capcom had publicly promised that the PlayStation and Saturn versions of X-Men vs. Street Fighter would both be arcade perfect. The PlayStation version, released in Japan on February 26, 1998 as the EX Edition and in North America that June, was not, and the reason given at the time and repeated in the record since is plain: the memory limitations of the PlayStation. A tag team game needs both partners, fully animated, resident in memory at all times. The PlayStation build could not carry them.
So Capcom cut in stages. This page lists what went, in the order the record supports, and what PlayStation owners received in exchange.
THE TWO COVERS
Same painting, two very different games

WHAT WAS CUT
In the record's own orderWHAT THEY DID GET
The PlayStation version's own additionsThe PlayStation version was not left empty handed. It added a Training Mode and a Survival Mode that no other version of the game has, and its Hyper Combo Gauge fills in roughly half as many hits as the arcade and Saturn versions, enabling more frequent combos. Game Informer and IGN both held that, judged on its own, it remained a solid fighting game.
The rest of the press was harsher, and the split is part of the record. GameSpot scored it 3.6 out of 10 against 7.4 for the Saturn import, Next Generation gave it one star out of five, and Electronic Gaming Monthly's team split between decent fun on its own merits and slowdown that killed any enjoyment. The aggregate sits at 64 percent on GameRankings. The tag team was the game; a version without it was always going to be judged as a fraction of the original, and it was.