Caveat: I don't keep up with Smogon tiers, or participate in competitive battling.
>What makes it unhealthy?
It would depend on how exactly you are able to counter Mega Sableye. Just by looking at him, his typing is very problematic for a hardcore defensive stall Pokemon, because his only weakness is Fairy, the newest type that has a really narrow movepool. Sun/Moon seems to be changing this, as there is evidence that Fairy will be gaining Super Effective against Ghost, making Mega/Sableye to take 4x from Fairy, but that is a small favor without a pretty substantial expansion of its move list, as well as species that can access that move list.
The other issue is that, for a stalling Pokemon, Mega Sableye has quite usable Attack and Special Attack. 85 isn't spectacular by any measure, but it's decidedly not bad either. Its quite usable. This is a bit of an oversight for a Pokemon intended to be defensive in nature. Not to say that he should have something silly like 20 or something. But I'd think somewhere in the 55 to 65 range would have been smarter.
You are right, it does depend on how you look at it. But it also depends on the meta as a whole. We can already surmise that Alola will shake up the meta as it is. Alolan Ninetails in particular may well solve the MSableye problem (Ice/Fairy instead of mono Fire), and we don't have stats for any of the new Pokemon. I can only hope that Smogon will hold off on making any Tier changes until after Sun/Moon comes out and has a little time to settle.