Magna and Monolometrodon usually lose if they don’t KO their opponent in 2 hits. If they do, you get turn 3, which is what happens in most of their favourable matchups, and the favourable matchups are the reason you’re using the creature in the first place.
Monolometrodon has more HP, but Magna has more damage, critical chance and speed, so there isn’t much of a difference if you’re trying to boost them to the same stats.
Regardless, I don’t think the bad turn 3 is going to get people to drop it unless they have something better in that regard (i.e. Magna), and low level players obviously have nothing better that does the same stuff, so I think it’s irrelevant outside of the endgame, but that doesn’t mean it’s a bad point in general.
It’s irrelevant in lower arenas, but not for the reason everyone seems to be saying it is.