Hippo, Gastro, Amoonguss.... these are the kind of pokes which are somewhat 'tier immune' pokes. That is to say, they will do their job remarkably in every tier; but they have some flaws that will always stop them from being broken in any tier.
I guess people do not want to provide the high amount support that pinsir-M requires compared to other megas like gross and altaria. Speaking of Gross, its rise has also been very problematic for M-Pinsir, since M-Gross outspeeds and resists quick attack.
I doubt Mandi would be too significant in UU. Its builky as hell, but does not have much in the way of actual resistances. Alakazam moving up means thats one more thing Mandi could have but will not be getting the chance to. Still a pretty bulky defogger who can provide solid utility.