Ok, so even though Mega Scizor is pretty much better than Mega Aggron 90% of the time, Mega Aggron does have some useful niches. It can tank +2 Flare Blitz from Talonflame and then KO with Rock Slide, or even live +1 Adamant Zard X Flare Blitz from full and retaliate back with EQ (which actually has a chance to KO factoring in recoil). Aggron is also able to set up rocks and phaze while being able to beat fairies, which is something Garchomp can only dream of.
Sure, it may be usually outclassed by M-Zor, but saying Mega Aggron has no niches is really underselling Aggron's capabilities.
Klefki can't set up rocks or beat Ferro with Fire Punch, shuffle, etc.TWave is a justifiable niche but when TWave is also available to many Pranskter users and other Walls it isn't a singular niche of why to use Mega Aggron. Klefki comes to mind as a Steel type that can use TWave to great extents
Sure, it may be usually outclassed by M-Zor, but saying Mega Aggron has no niches is really underselling Aggron's capabilities.