MegaZam, only having 65 base defence and 60 hp, takes only 50% from technician stab boosted Mach punch from Breloom, a pokemon who likely has good investment in attack IVs? I'll take that for comparable attacking stats to the special side of Deoxy-A.I really hope you're joking about MegaZam. It dies twice as fast as normal Alakazam (no Sash), has marginally more power than normal LO Alakazam, which doesn't make a difference thanks to Magic Guard. Since it loses Magic Guard, MegaZam basically is asking for Scizor to come in, and if Scizor continues to be in 20% of teams, that's not a good number of games you are practically down one mon in. Also it takes about 50% from Breloom's Mach Punch, haha. I'd rather use Mega Heracross, for goodness sakes. And we all know how good that thing is...
Especially when you consider it yoinks any ability on its way in and with 175 base special and outspeeding anything barring the literal top 3 fastest pokemon, I feel like MegaZam is in a much better position to sweep a team than MegaGar is since, as both have general use as revenge killers and neither has a stable way of locking in the opponent upon coming into battle, Zam has the advantage of good coverage and (albeit marginally) better stats for what its purpose is.
Scizor is going to do what, its only option is technician bullet punch and if that doesn't score the kill its as cooked as bacon. Once that's out of the way you'll only be able to depend on any other kind of priority you may have. Just imagine MegaZam stealing some of the abilities that might pop up in ubers... Lord forbid he manages to steal your Speed Boost, Multiscale, Sheer Force, or Sturdy. Or even, say... Shadow Tag. That's one pokemon you don't want to have to switch in to.