I actually just bought a P65+ (used) and a RZproIID in 2016 and L O V E it. It is quite a change from the Nikon D810 but for the better. Feels more like real photography.
The P65+ has been super realiable for me and no tethering issues. Just always shoot with a battery in the back, never rely on the power supply of the FW port in your computer. Having said that the P65 is much like shooting Film - you are more or less limited to iso400 and it behaves the same in many ways. A modern DSLR (and the files it produces) like the D810 is technically superior but I still prefer shooting the RZ.
The RZ IID has been pretty reliable with the occasional hiccups - most of the time taking off the adapter plate and putting it back on solved the issues. I had some shooting days with several hiccups but most days are without any issues.
From what I gather from my friends I would not reccomend shooting on the RZII (without D) as you need a lot of funny synccables to make it and it that's quite prone for error.
Shoot me an email if you have any questions:
dave.muc@gmail.com. Also I can send you a PDF with custom masks I made.