I had an Ultegra 12-30t with short cage mech and it worked perfectly. I switched to an XT 11-32t cassette and, while it worked sort of OK on the bike stand, I wouldn’t have trusted it out on the road, so went for the medium cage mech and again it works perfectly.
Question: would I get away with an11-34t cassette with the Ultegra medium cage, as I live in hilly West Yorkshire and have serious knee problems. (Sorry, not trying to hijack your post, OP!)