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Why don't you tell me about Mega Edgedoge/Houndoom?
We know it suffers from "it hits hard but dies too quickly" and arguably requires sun up 100% of the time.
Its speed tier is not bad at all (115 if you don't know it) and 140 Sp. A that can be boosted just by having sun is bretty gud. Consider STAB+Solar Power+Sun boost and its Fire moves become quite ridiculous.
Fire/Dark is a nice combo that I think no other prominent mon has, though it also makes it weak to common attacking moves (very common attacking moves) so it's understandable in that regard.
Also people seem to think that it's mandatory to run it with Ninetales, adding more to the Fire weaknesses but that's up to the player and I believe it should be looked into. Same way it's not mandatory to use Politoed with Mega Swampert. It's by no means dead weght if you lose the sun during the match (but of course you want some way to have it if you pretend to use this mon in the first place).
Anyways while what I said sums up most of it it's always good to hear others' opinions on this.