It's different in Tekken, but it still matters. As Saint and JDCR said, it messes up their movement, so they are afraid to do shit like sidestepping. And as you know, movement is a huge deal in Tekken. So altering your gameplan around the input lag trying to avoid linear attacks and getting caught on them is a big deal.
But I've seen people downplay it. "Oh SFV is 8frames of lag so it isn't as bad." Motherfucker do you want your game to be better? SFV gets a lot of shit and a lot of memes, but when it comes down to other games fucking up, it's "at least it's not SFV" which is a horrible precedent to set, even if you hate SFV. You want your game to be the best it can be. Don't sugarcoat it just because you want your game to seem immaculate compared to SFV.
Same with Tekken 7's awful online connections for the PS4, it's barebones online options, and lack of content. I want every fighting game, SFV, Tekken, Gundam, and all the rest included, to be the best fighters they can be. And all the shitflinging at one game prevents honest discussions.