It has bad stats across the board and it has to attack in order to morph. The other starters can just wallbreak everything. Mega Eevee can’t do that. It has limited TMs and can easily be outsped and killed.
It can learn a powerful STAB move for each of its evolutions from a special move tutor. Flare Blitz, Hydro Pump, Thunderbolt, Psychic, Foul Play, Leaf Blade, Ice Beam, and Moonblast.
Already, that’s quite a repertoire. Beyond that, every time it uses a move, it transforms into the Eeveelution that gets the STAB boost, stats, ability, and all. Yet it still keeps Protean Maxima running in the backround, which means that while it does now have access to that form’s ability, it can change into any other at will.
Also, assuming that we get the Event Move Tutor later on, like we had in Z/O, Eevee will also have access to Extremespeed, quite the perk.
So, it can change it’s type, ability, and even stats (including HP) basically at will. This thing has a lot more power than you give it credit for.