What are the consequences of giving an ASI or feat every 4 character levels instead of every 4 class levels?

I’m trying to figure out why Dungeons & Dragons 5th edition designers decided that ASI or feats would be given every 4 class levels and not every 4 character levels.

If I allowed my players to get an ASI or feat at character levels 4, 8, 12, 16 and 19, how would the game balance be concretely impacted? How could the players abuse this?

The peculiarities of some classes such as the fighter and the rogue would be kept: a rogue would still gain an ASI at rogue level 10 and a fighter at fighter levels 6 and 14.