The RZ lenses have electronic shutters that are controlled from the camera body. That is a plus, but makes it impossible to use them on a body that lacks that electronic control.
And at least some of the finders interface with that electronic control.
The RZ67 was designed to replace the RB from the get go. No doubt whatsoever. It was only later that they finally realized people were going to keep buying the RB (for whatever reasons) and decided to keep making them. I am sure they would have preferred to not bother, probably. But the money was too good, I'd guess.
What functionality was lost? None that I can think of. Everything was/is a gain - which is why it was designed as a replacement. the RZ is a superb system, and especially these days with the crazy cheap prices, no reason not to choose it over the RB. RZ lenses are actually cheaper in many cases than RB lenses (more of them available, the APOs for example).
For my purposes, I (generally speaking) have chosen not to buy cameras which require a battery and chose the RB67 Pro-S for that reason. In hindsight, I wish I had chosen the Pro-SD, but I'm very pleased with my Pro-S all the same (even the 120 backs which need to have the light-seals maintained). My 35mm gear all requires a battery for some level of functionality (whether just for the meter or for both meter and shutter), but I tend to prefer the least amount of electronics for my film gear.
I got a #2 (82mm) RB extension tube in the mail today. I will experiment with it using my 127/3.8 on an RB67 Pro S. A 180 is on its way here. I will try to decide whether to get a 65 after that or a 140 macro. For hand held use I prefer my Bronica GS-1s but the RB is much better for closer work.
I'd recommend the 140, as it not only gives beautiful close ups, but without the extension tube can give stunning landscapes too.