Did I miss something?

The biggest difference between a disk cylinder is that it does not have residual valves in the drum side, and most have a larger "tank" on the disk side.

Neither of the above is the issue

What is the issue is whether the hole in the back matches the linkage properly so that when the pedal is up, the piston(s) return past the compensating ports. If they do not, the brakes will lock and stay locked, or at least drag severely

For example, I just converted my car to 4x brakes. I'm still using the original 67 drum master. All I did was to poke a nail into the master fittings to rupture the residual valves. The thing works just great. It just happened to be a good bore size to give me good pedal effort.