Wonderful potential squandered by poor mechanics and design choices. The two largest flaws are the lack of holding an item and the flat 100 BST boost. The former means a lack of variety in sets you can run and roles they can play, the latter means that some megas like the pseudos are better than box legends while mega sableye for instance is weaker than plenty of ordinary pokemon (with no item to boot).
personally, Scizor and Scyther's relationship in generation II. A type change, a stat redistribution that changed how they function in battle, and a visual redesign the complete opposite of just gets bigger. To me Pinsir is the gold standard for a mega evolution. Type change, ability change, operates differently in battle than its base form, and a visual design that is clearly still Pinsir but taken in a new direction.
Mega evolution should have no stat boost compensated by the ability to hold an item. An opportunity for pokemon which aren't good but have a design and BST too advanced or high for an evolution to get a new lease on life with a new typing, ability, and rearranged stat spread. Pokemon like Sableye and Mawile have neither the design or BST of a fully evolved pokemon, and should have got regular evos.
