Alright, just going to nip the damn Sand Veil arguments in the bud right now. Don't you people forget I got my Poster of the Month award partially for pointing out just how ridiculous a ban on that would be.
1/5 is not a huge chance. the vast majority of the the time, SV Gliscor just gets wasted before it can do anything, or Phazed out (and with no recovery on pretty much all SV sets, you can actually KO this Gliscor with SR damage). Defensive Pokemon wall it handily after you've taken the air gem boosted acrobatics (It's one shot then goes to being a fairly normal move remember). Anything with Ice Beam that can either outspeed or take a hit will OHKO it 80% of the time. Substitute just makes it die from residual damage faster too. Sure you CAN get terrible luck every now and again and have it miss like 4 times in a row, but you can get the same luck with missing Stone Edge, or Focus Blast, or crits, or defense drops, or any other of the gajillion things that hax in Pokemon. Sand Veil just gets singled out because it's not as common as those things are so we remember it more.
Not to mention it barely impacts the metagame at large at all. SV Gliscor is hardly ubiquitous. Sand Veil is used on all of 16% of Gliscors last month according to the lovely stats Antar has provided us with, and that amounts to a whopping 2% or so of teams using it. That's not even enough to stay OU people!
Sure, with a bit of momentum from hax it can ruin a team, but so can ANY goddamn Pokemon. How about if Haxorus crits your Skarmory and that's your only steel? How about if Alakazam scores a Sp. Def drop on your Blissey? Gliscor may get this type of hax more often, but it also needs MORE of it to be successful due to the fact that it has, y'know, two type coverage (assuming substitute) and 95 base attack. It's hardly unfair; we just get pissy whenever we get haxed out of a win, so naturally a Pokemon that wins by haxing is going to get us extra pissy.