That’s exactly the difference I mean when I say the definition most people on SRK is too narrow. A comeback mechanic can (and usually should) also benefit the player in the lead. The classic example would be Jeopardy, where everyone gets more money for each question the further into the game they are, and if it isn’t a complete blowout, the losing player can bet everything at the end of the game to try and take the lead.
In Street Fighter, there’s usually fairly good defensive options for whatever your opponent can do to you once they take the lead, so barring a few exceptions, you can probably get back to neutral, where you don’t have any real disadvantage other than having less health. In HF, the randomness could always swing in your favor, and generally in SF2 if someone made a big mistake it didn’t matter who was winning, you can fuck them. Once the super move was introduced, you can sort of see how that acts like the rounds in Jeopardy. Later into the round, both players are likely to have meter, so the plays that happen then are generally going to be bigger. I feel like the devs of SF4 probably felt that all the ways that you had to think about meter in SF3 led to almost exclusively complicated game states, so they just generally made it simpler and added Ultra to compensate.
KoF even more so. There’s a lot of back and forth, and you only really see big plays going on when someone blows a fat chunk of meter.
GG and BB aren’t so agnostic to leads. Once someone is on you, they’re going to be doing all sorts of nasty stuff that just doesn’t happen in SF. So, you have burst to let you have a chance to create your own opening and take the lead just like you would in 3S or whatever, plus everyone has meter, etc. There’s also the thing in BB where you get free meter when you’re low on health, but that’s not exactly X-factor.
Compare all of those to, say, Chess, where if you fall behind by three pieces, you are probably proper fucked. It makes up for that by giving both players perfect information about the state of the game at all times.
Marvel is sort of unique among fighting games in that falling behind actually makes you worse. So I feel like if anything would actually benefit from some kind of asymmetrical boost to the losing player, that would be it, but I feel like basically everyone in the world thinks that X-factor is, if nothing else, a ham-fisted way of going about it.
Basically this.