I was considering slow-cooking pork with pineapple, but was concerned on the effect the pineapple might have on the texture of the meat. I understand that both bromelain and acid can deteriorate the texture of meat. Is the concern legit in this case?
Bromelain, at least, won't be a problem, for two reasons. First, although bromelain exists in all parts of the plant, the enzyme exists in larger quantities in the stem. This, plus the fact that the stems are left over after harvest, are why the enzyme is typically extracted from the stem for meat tenderizers.
Second, bromelain is heat labile, which means that heat will denature (deform) the protein and reduce or eliminate is enzymatic activity. Slow cooking should take care of that.
So that leaves the acids. Acids, in general, also denature the proteins of the meat, which is why they are frequently used in marinades, though some contend how useful they are in that respect. Pineapple pH is 3.3-5.2, which isn't too bad. For comparison, lemons are pH 2.2-2.4, and since that's a logarithmic scale, it means that lemon juice is at least ten times as acidic. Since you're also slow-cooking, all things considered, I don't think it is going to have much effect.