More importantly, there's a difference between "primitive" and "buggy" when it comes to medicine. If DF 31.xx's medicine was primitive, i.e. doctors using leeches and experimenting with mercury (which really happened!) and other crazy stuff without really understanding how any of it works, and hurting dwarves as often as they help, I could understand that. Doesn't sound like it would make for a fun game, but it would be believable - that's not the problem, though.
DF 31.xx's medicine is not primitive, it's buggy. I've got Doctors refusing to even diagnose patients until you deliberately give them a new injury to trigger treatment, dwarves completely ignoring any finger/toe injury until it gets infected and causes death (I don't care how primitive your medicine is, it should be common sense to bandage a wound or clean it or try something), nerve damage which happens too easily and never heals leading to frequent permanent paralysis, dwarves with moderate injuries (broken limbs, "torn open" tissue, etc) sleeping for years even after the injuries have been treated and healed, etc etc.
I mean, I love 31.xx (for the vast underground/caverns especially) and I don't think I could go back to 40d, but how I hate the medical system. No matter how well I set up my hospital, I have to write off a dwarf that gets any injury - many of them will refuse to go to the hospital for potentially infection+death-causing injuries, or won't get treated despite living permanently in the hospital, or won't ever recover despite getting treated and healing, or.. I see recovery as a miracle, great when it happens but never to be expected.