Tangs in the wild swim miles and miles a day. Because of this, their slime coat is inherently light, to help glide through the water better. The indirect problem, is that their thin slime coat, leaves them extra vulnerable to Ich infections. It's not the Tang's fault many hobbyists experience Ich problems, it's the fact that we're keeping sensitive creatures in closed systems. In the wild the Tang can just swim to an Ich-free location. in your tank, there's literally no-where-to-run.
If you're only planning to get your fish from ***** or a sleazeball LFS, and don't plan to QT, stick to damsels and skip a tang.