I have a sous-vide supreme water oven. I was wondering if I could use it to slow cook large amounts of soup or to make beef stock by slow cooking. The idea is to have the soup or stock directly into it and no bags. As it is easy to clean and is perfectly sealed, I do not foresee any problems but I am having cold feet. Could somehow I manage to ruin my oven? What is the worst case scenario here? Any ideas?
You certainly could.
I read the PDF manual online at the manufacturer's website, and it appears that this particular machine has no internal circulation device like a water jet or impeller (somehow, fan doesn't seem like the right word under water). It relies entirely on thermal convection to circulate the water bath. In fact, it seems to have no moving parts whosoever. So there is no seal in the bath containment through which ingredients would leak out into any machinery, lowering lifespan.
According to the manual, the bath container is not removable, and the entire device should not be put under water. This means that cleaning it out effectively is not actually that convenient, at least to my mind. Mild soap is permitted, and the bath is made of stainless steel so it could probably stand up to much harsher treatments--the manufacturer just doesn't expect you to need them.
Worst case, you have to spend time carefully cleaning the bath, then tediously transferring water in and out to rinse it. I cannot see any way you would harm the device itself.
For the applications you have named, you don't need the precision temperature control this device provides. You could do them just as easily in a traditional slow cooker (whose crock should be fully removable for cleaning), or in a normal stock pot on the cook top.
And they would be much easier to clean. And you could brown the stock ingredients directly in a pot with the second method.
So just because you could do it, doesn't mean it is the best method available to you in my opinion.
Don't take the risk. You probably could use this for making stock, however what if some part of your stock burns on it? It's an awfully expensive piece of equipment to risk, when you could pick up a used crock pot for peanuts. Plus, it's going to be a pain to clean out, and it will need a load of cleaning after you make stock in it.