You can overcook anything with a variety of methods. A slow cooker has no monopoly on this.
As has been pointed out, some cuts of meats do well with a long, slow cook. Others, not so much. To maximize flavor, you should brown the meat before adding to the slow cooker (along with any accumulated juices). This is the same as with any other cooking method.
I have a slow cooker with a metal insert that can brown the meat before changing to the slow cook function. I think I do more b/s chicken thighs in the slow cooker than any other method (other than the grill). I brown the meat (and sometimes other ingredients) and then switch to the slow cooker function. Cooking time is about 2 hours on high, 3 hours on low. The meat is fork tender and very flavorful.
The point is that you have to choose the method appropriate to what you are cooking. Slow cookers are good for tough cuts of meat, hopefully with some fat marbling.
That said, I think that in a lot of cases a pressure cooker will produce superior results. The convenience of a slow cooker has some advantages, though. You can monitor cooking and make adjustments in a slow cooker, but you can't in a pressure cooker.