Zard Y needs to have a clear field - if stealth rocks are up, it has trouble coming in. Excadrill takes next to no damage from Stealth Rock. Furthermore, Zard y's speed tier is rather average. It functions as a wallbreaker, whereas Excadrill is not only a sweeper but has strength that is close to a wallbreaker.Oh, i understand better your point of view now!
But (because it always had a but), there is the same problem with some type against M-Zard Y (Grass, Bug, Ice), M-Pinsir (Grass, Fighting), M-Medi (Normal, Steel),...
What about this? They aren't Sandstorm dependents...
Mega Medicham is strong, sure, but we're back to the issue of speed tiers - it does have a few priority attacks, but they're not all too powerful.
Mega Pinsir is a different case - strong, access to SD and great coverage, but suffers from Stealth Rock which bug may struggle to keep off the field and does rely on quick attack at times.
They may not be sandstorm dependent, but they can only hold their selective mega stones. Excadrill can hold the Life Orb, powering its attacks up even more. Also, it outspeeds everything in the meta under sandstorm, which is the issue.