The thing about Sand Veil and Snow Cloak that makes me support a complex ban on Sand Veil + Sand Stream and Snow Cloak + Snow Warning on the same team is that you can eliminate someone's check to your Pokemon with just one miss. One miss. Since these abilities increase Evasion by 20%, it lowers your typical move to the same accuracy levels of Stone Miss.
Let's look at a very possible scenario from OU before Garchomp's ban. Garchomp comes in on your Blissey. Blissey will be taking a lot of damage from an Outrage, so you switch out to your Scizor to start Bullet Punching it, but Garchomp instead subs. You Bullet Punch the sub, it hits, breaks the sub, Garchomp Earthquakes, doing 47.52% - 56.27% to your CB Scizor. You go for Bullet Punch again, but what next?
You have a 64% chance of hitting an 80% accurate move twice in a row, which is actually, in my mind, very low.
Oh, but this can happen too!
You switch to Scizor on a predicted Outrage, Garchomp subs instead. You go for a Bullet Punch, and it misses. Garchomp goes for the Earthquake, for a solid 2HKO after Stealth Rock. You now have no hope of beating Garchomp with your Scizor, and there is also the possibility of Bullet Punch missing again, leaving you with your Scizor down (and you have no Ice Shard, so no priority) and going up against a Garchomp with a sub up. Good luck.
You could have just kept Blissey in to ST Garchomp, breaking its subs - Oh, I almost forgot, you also have that nasty chance of missing Seismic Toss.
However, while outright banning Sand Veil and Snow Cloak would theoretically work, it is not a viable choice because this will lead to a soft ban on several Pokemon that are clearly not threatening, such as Glaceon, Froslass, Sandslash, and Cacturne. This is why I support a ban of Sand Stream + Sand Veil and Snow Warning + Snow Cloak. This method effectively reduces the issue that is Sand Veil and Snow Cloak, makes the metagame more enjoyable, without removing perfectly viable Pokemon from the lower tiers.
Also, I saw an argument that "Stone Edge and Hustle do the same thing". I don't understand this logic. Sure, Stone Edge in no weather has the same accuracy as Close Combat vs a Sand Veil user in the sand, but you can actually do something about Stone Edge's accuracy yourself: Stop using Stone Edge. What can you do about Sand Veil? You could run your own weather, but that's not what I call reliable since you can't guarantee that you can keep your weather starter alive long enough to keep Sand/Hail off the field.