I gotta say, I agree with Jas, Jimera, etc.. Garchomp is really the only thing that uses Sand Veil ever, since Gliscor mostly uses Poison Heal, Dugtrio is unviable without Arena Trap, Golem sucks, Donphan needs Sturdy, Sandslash is a Sand Rush kind of guy, Stunfisk is WTF, and then Cacturne is alright... but really rare. So basically, its pushing back Gliscor (who doesn't even use Sand Veil) and Cacturne (who is rare, and would really not appreaciate losing Bullet Seed or Encore in every tier). This is almost aeshetical, really, and it hardly changes the metagame when you think about it.
My problem is this ability is going to be banned just because Garchomp wants to go back into OU. This isn't a good precedent, really. If you think about it, it it kind of like Speed Boost Blaziken. If Speed Boost was banned, Blaziken would come back (in RU or something), but then Yanmega would lose a lot of steam and Sharpedo would lose a huge option. This is similar for Sand Veil. Cacturne would lose compatability to huge options, which puts it at an unfair disadvantage compared to other Pokemon in lower tiers.
As for it being in Evasion Clause, this is true. However, with Rain being on over 20% of teams, and Hail or Sun making up another 10% or so, this isn't huge (sand is at about 25% usage iirc). What this has compared to Moody, Double Team, or something is any of these Weathers can just straight up stop the Evasion from happening. Even if you don't have any weather, if Garchomp gets up a substitute, you always have priority, Choice Scarf, or Multi-hit moves (although Breloom cannot live to tell the tale, still).
Really, if an ability is going to get banned just to move ONE Pokemon into OU, either a complex ban should be implemented, Garchomp should be OU ignoring Sand Veil, or it should just stay uber. Plus, other than Garchomp, does anything really say "OMG IT HAZ SAND VEIL ITS UBER STRONF BAN IT NAO PLZ!!111!!11!" because the usage just doesn't tell me the ability is really that prominent.
That, and Sand Veil in general has always felt like more of a feeling to me than real madness. I haven't ever seen a Sand Veil user in OU after the chomp ban (I've used Chomp in Ubers with Sand Veil. It may have activated once for me, which is low considering I've used it four times).
EDIT: Jesus Christ there were a lot of posts when I waas typing this.