We use ground Chuck for the meatballs when making meatball subs. No spices or chopped onion, just meat and the meat sauce is a basic tomato based concoction. Then we cook the meatballs(raw) in the sauce in a crackpot for about 10 hrs! When we serve it, the sauce has cooked away by 95%. What I need to know is can you overcook meatballs and/or sauce (on low setting)? They always have a terribly strong and bitter taste that ground beef shouldn't have.
That long time in the crock-pot sounds bad to begin with. Ground meat should be just cooked to a safe temperature -- any more and it will lose moisture and get tough.
The bitterness could come from prolonged exposure to the acidic tomato sauce. If you've ever made ground taco meat and over-seasoned it with lemon or lime juice, it will often taste bitter or astringent.
Bitter flavors sound like something is burning. Your description indicates the sauce is reducing by an extreme amount. It seems possible it is burning on the bottom of the crock, and the burnt flavors are infusing throughout the dish.
You are not describing a method which is likely to produce outstanding results. The lack of seasonings, breadcrumbs or similar (which change the texture to a more... well... meatball... consistency), the lack of browning (to develop beefy flavor), as well as the possible over reduction of the sauce all seem to lead towards a poor result.
The 10 hour cooking time may be the least of the issues, although it is longer than required by a considerable margin. Even so, if the sauce doesn't over reduce, and the temperature is well maintained at a very low simmer, it should not be excessive.