It's not about the stats, though I'm not a huge fan of the slow and bulky route they took with it. It's about the aesthetics and the ability. Mega Heracross is quite literally (in my opinion) the ugliest mega in existence, which is surprising seeing as it was spawned from (in my opinion) the coolest looking pokemon.
In 5th gen, Heracross was infamously known for one thing in particular, hitting fast and hard with a choice scarf, while maintaining reasonable bulk. Does mega Heracross follow that same style at all? No. Instead it hits like a truck, but struggles to out speed much while being really bulky. Additionally, before 6th gen Heracross' only multi hitting move was fury attack, but they give mega Heracross skill link and a plethora of moves to abuse it with.
In the end, all this feels like is gamefreak thinking "we need a skill link mega, I know lets use Heracross" rather than actually thinking about the role that Heracross has played for years and building off that. As a result, mega Heracross doesn't feel like mega Heracross, it feels like a completely different pokemon all together. A pokemon that I don't particularly like, and the worst part is that there's no second chance for him.