It's defensive stats aren't bad, admittedly. But Grumpig has a much better defensive typing (Psychic isn't a particularly great type defensively, but Ice is just about the worst you could ask for), better defensive ability, and a better defensive movepool.
As for Stealth Rockers that can scare off Glaceon...granted, there are few. The usual strategy is to set up SR ASAP, however, so trying to switch one in on Glaceon isn't much of a problem. On the other hand, Rapid Spin is a bit harder to block in UU, with no real defensive ghosts (Drifblim doesn't count as defensive). So...once again, while it's difficult to find a 100% sure way to beat Glaceon in a situation like this, I'm of the opinion that Glaceon should be tested in UU before being placed in BL. Despite it's obvious power, it still isn't particularly difficult to kill, and there are plenty of pokemon out there that can take a hit from it and either KO or do heavy damage in return.