They also have a loop that just wins turn 1. Even without that loop, a 6Sam going first with a turn 1 Gateway can easily get out LSS Shi En, Steelswarm Roach, and a bunch of other Six Sams as fodder anyway. Seriously, how am I supposed to win when they get LSS Shi En, Shogun Shien, 2 other 6Sams that pop S/T and Monsters as well as a backrow to counter anything I can play, before I get my first turn?
Also, I'm opposed to most cards being Limited or Semi-Limited in general since imo it makes the game more luck-based, as whoever gets their singleton limited card (like Monster Reborn) is at a big advantage, and only one player getting their good limiteds in a game is really common. Gateway is like the pinnacle of this theory; with Gateway and decent other cards in their opening hand 6Sams are nearly unbeatable, and a hell of a lot better than without. That makes it really luck-based whether you have a playable matchup against them or not.
Certain cards I'm okay with having limited or semi-limited, but those are usually due to them being either a) easily searchable (see Synchros/XYZs or those in archetypes with good search/draw support) or b) significantly stronger in multiples for whatever reason (most of these also have to do with the first point as well, like Reborn Tengu).