I think people have actually mentioned two issues a lot of people have with horizontal progression.

Firstly additional progression that gives minor stat boosts over a long period of time ends up adding up to a substantial difference which creates a major obstacle to new players and alts. Catching up becomes extremely daunting.

Realistically Horizontal progression needs to mainly be either non combative or situational ie. bonuses work only against certain enemies and in certain content. An example of situational for example would be GW1 where the reputation with the Order of Whispers had a progressive bonus to damage and reduction to damage received from a particular group of enemy. Similarly they offered other horizontal progression where players gained access to skills that were potent but specifically against enemies of a certain type or in certain content. This way a new player only has to progress on content that they and their friends are doing now rather than scaling a mountain from the very beginning.

Second issue is multiple gear sets. While if you have one or two Jobs this probably isn't an issue. If you have all 21 however it quickly becomes an inventory nightmare. Further having to constantly switch around gear to do different stuff is annoying and requires a hell of a lot of time building up different sets. FF14 struggles with its inventory situation as it is. A multi gear set situation just sounds completely unfeasible.