My experience with tangs has been pretty weird. I'm not a big tang fan, so I've only kept a few. I've had a Lineatus for 2 years and a PBT for 4, in separate tanks. These are both fish that are considered 'expert' level. Hard to acclimate, get to feed, susceptible to disease, etc. Both of these guys have been the most reliably hardy fish in the tank for me. Sometimes when water quality will slip a bit if I'm being lazy the tangs are the last fish to show it. No HLLE, great color, nice and fat. I should point out that both are the only Acanthurids in the tank. I should also point out that out of 5 people I know that got a Lineatus from the same shipment as mine, only one lived longer than 3 months.
If I had to guess, the reason they're considered so difficult is that they don't handle those first 2-3 months very well. Housing one with another Acanthurid in a small tank during that time is an all too common mistake, IMO. They may not kill each other outright but the constant stress is a factor to consider. Once you've got it feeding and it's immune system back to normal I think you're golden. But that's just my experience, and it could be atypical.