>>21614120No shit Pokemon have weaknesses. No one's saying it's the best thing in the game.
However it has no true counters. It has checks, sure, everything does, but even what you listed isn't a counter.
A counter has to be able to come in, take any hit from it reliably, and threaten to KO it back whether it be through means of outspeeding or tanking yet another hit.
M-Salamence's movepool is just simply way too big to be able to choose a set check and reliably work with that. Even more so, it basically fucks up anything that ISN'T a set check (which is an element /vp/ often overlooks when determining broken pokemon). Your standard team has to specifically sacrifice team structure for a check to it, otherwise it'll get steam rolled by it. It's extremely difficult to outplay (assuming both players are of equal skill), which forces specific team building moves, which is in turn over centralizing and therefore broken.
Centralizing is fine. If your team happens to be weak to Heatran then adjusting a few things to stop it is a natural part of the experience. Over centralizing, where you NEED a stop to it or else your team will be torn apart, and then the meta just turns into M-Salamence cores+it's stops, is not fine.