Slow cookers are only good for braise cuts: beef chuck, chicken thighs, pork shoulder. You want to put some liquid in, but not too much. You want aromatics (a whole head of garlic, an onion). I never put starches in my slow cooker anymore.

The low-temp water bath is definitely much more precise. You can't cook a steak in a slow cooker; the cooker runs near the boiling point of water.