I recently read that so-called experts say to never put frozen meat into the crockpot; meat should always be properly thawed before cooking. I haven't been able to find a reason for this and, of course, they don't say. It's just a blanket statement.
I almost always put frozen meat into the crockpot. I don't even cook it longer than I would if it were thawed (because it's done) and I rarely add water. We haven't died yet!
The danger of frozen meats are that you could actually damage your crockpot. THE RULE - Never Preheat Your Crockpot (unless you are going to boil eggs or your food is already hot) - COLD CROCKPOT - COLD FOOD. HOT CROCKPOT - HOT FOOD. If you put cold meat in a hot crock pot or vice versa it may crack your crock pot.
Also, always gently place frozen food into the crockery. Otherwise it could crack upon impact. My daughter once dropped a frozen roast from the store package into the crockpot, not thinking that the frozen food is hard and the crockpot is basically glass. Of course, it broke in half.