I think the question is pretty self-explanatory. In many ways medieval medicine just seems crazy with nearly everything they are doing being either useless or even harmful. But I am sure there must be a few things they were good at? (Obliviously not in comparison to modern medicine) Maybe even more than I would think?
More than you're expecting. More can always be said on the matter, so anyone with knowledge of European medicine in the Middle Ages should feel free to contribute! Whilst waiting for new stuff, here's some previously posted stuff:
Bonus: u/BRIStoneman does link to it in their earlier answer, but for the convenience of anyone passing by, Bald's Leechbook can be read in all its digitised glory here.