Something I've been leery about lately when switching to upright bars is how the frame takes the new directions of stress. I've had a few taller frames exhibit some pretty noticeable frame flex when converted to IGH and North Road bar bikes. I never saw any real data on how rider position affects frame stress, but I did have a down tube break off at the BB shell on one bike with Champion
#2 tubing, and had a Schwinn Volare frame loose a right chainstay at the BB shell. both were tall 26 and 25cm frames.
I tend to look at 23" frames as the ideal upright conversion size for me because they're just a more rigid frame in most cases.