Well, no treatment is ever really 100%, but if done correctly it usually works fine. Coppersafe is a bit more effective, but has its own drawbacks.
Most cases of ich coming back can be traced to new fish being added which were not properly quarantined.
Interested in your thoughts. I've always read that hypo is very effective but only if you keep your salinity at a very specific low level (1.009). Any lower and depending on the fish you can have issues. Much higher then you risk not being effective. The reason I have always stayed away from hypo is that most people QT in smaller tanks without proper ATO's. It always seemed to me like it would be a pain to keep it at therapeutic range...I mean doesn't take a lot of evaporation at smaller tank volumes to cause an increase in salinity.
Meanwhile, copper while having its own set of drawbacks has a pretty wide range of safe and therapeutic levels. You need a good tester and to be able to get the fish out of the display, but otherwise is a safer method...by safer I mean less likely for a novice to screw up.
To me hypo is a niche method. Either for copper sensitive fish or FOWLER displays. In most other cases copper makes more sense (but again not the only option).