There are a lot of problems with shock collars in general. The obvious is that they may be considered a form of animal cruelty and that is especially true if they are used excessively. Another problem is that they might not work.
Believe it or not, strong willed dogs have been known to resist the shocks and do what they aren't supposed to be doing. For those dogs, one might argue that shock collars are downright cruel -- if the dog receives a bunch of shocks that don't come close to altering its behavior, that's where cruelty arguments come into play.
Still, there are some people who have found a shock collar to be the only way to properly train a dog.
The question of whether shock collars are effective, cruel or whatever else seems to be a discussion is hard to have because each dog is different and the needs of each owner are different.