Most tangs are more susceptible to ich than other fish. Although I have not heard of the clown tang not being recommended for this reason. I'm also thinking, if you wanted to buy it, and the LFS didn't recommend it, then why did they have that fish for sale in the first place? I don't think stores should sell animals that have poor survival rates in captivity in the first place. But like I said, I have not heard this about the clown tang. If your tank is established and has been healthy for a while, I don't see why not go for it. Just quarantine it first and minimize stress. Same you'd do for any other fish.