Mega Metagross also just doesn't die easily. At all.
Sure, Alomomola probably doesn't die to it very quickly but it does jack all back to you since it would be forced to run Scald just to weaken it, maybe, with a burn. And Toxapex is in the same ballpark but worse. Not only does STAB Psychic Fangs destroy you, but you have Earthquake in case you don't wanna run that. Failing that, Thunder Punch to at least maintain some super effective coverage. Again, all Toxapex and most waters can do back is fire off a weak Scald and hope for a burn.
To play devil's advocate though, Mega Metagross does suffer somewhat from four moveslot syndrome. An argument, not a particularly great one but one nonetheless, is that Metagross can be checked with enough bulky Pokemon. If it lacks Hammer Arm, Ferrothorn is an annoyance. If it lacks Thunder Punch, Skarmory and Corviknight are annoying as well with the latter draining your power points. Hilariously, you can use Hidden Power Fire to deal with these since your special attack is a respectable 105 but that's not the best (but the option is pretty epic.) Without Ice Punch, defensive gliscor and landorus become problems (the latter now able to both intimidate you and weaken you further if it holds Rocky Helmet.) Without Psychic Noise, Dondozo becomes a nuisance and even if you have thunder punch is quite bulky. Against defensive teams, an argument could be made that you can withstand the assault given the right partners and assuming Metagross lacks the correct moves.
However, my counterargument to the above is that because Metagross CAN in fact have the move to beat almost any check, it exerts a tremendous strain in the teambuilder that would most likely warp the tier around it. It would easily become S ranked, no question. The strongest all out attacker in all probability. And if that isn't enough, against offensive teams, forget about it. The moment that Metagross gets a free turn the game will likely snowball since it has the stats to wipe out a vast VAST majority of frail offensive targets that probably get OKHOed at worst and irrevocably damaged at best.
Oh yeah, did I mention it gets Grass Knot too? Yeah, bye Dondozo.