It sounds crazy to say, but Mega Gallade is actually underwhelming in practice from my experience. The main issue is of course Sableye and Slowbro, who completely cockblock any wallbreaking capabilities. Max defense Mega Altaria, although uncommon on stall because of sab and bro, completely cockblocks Gallade too. Hell even the bulky DD set which is quite common on balance can avoid the 2HKO from ice punch (if you carry it) and setup in your face. Swords dance builds are no problem for stall either with unaware clef and even unaware quag (close combat only has a 30% chance to 2HKO).
Everyone hates the opportunity cost argument but if you needed a wallbreaking mega why would you use Gallade over much better choices like Gard and Zard Y? They actually beat common stall megas, especially Gard, and even for hyper offensive teams there are better picks for the mega slot like Metagross and Lopunny.
Then there are the other obvious issues like getting destroyed by Talonflame and even quick attack from Mega Pinsir (unboosted does around 73-90 iirc).
I hope that didn't come off as if I think Gallade is bad, not even close, but it's really not that great. I've had success with it but I've found myself wishing I was using a different mega more than once.