I have 2 RB67s and an RZ67ii and have used them fairly extensively in the great outdoors. If any measure of being "professional" equipment is durability and reliability, I can only sing the praises of the RB67s. They are TANKS in more than just weight and heft, but they just keep going and going and going.
I have not had the RZ67ii as long but I have had it extensively in the field too. Any so-called "failures" have not been the fault of the camera at all, but rather the have been the fault of ME, the operator! A quick check of the manual discovered that the various interlocks and safeguards were working to prevent me from making mistakes. It was MY LACK of training that was the problem - something I continue to work to remedy.
For both the RB67 and the RZ67ii, if you take care of them (i.e. adjustment, maintenance, cleaning), they will return more than care for you.
As for the Sigma Six design hoo-haw, let's put this in perspective. These cameras were designed some time ago before Sigma Six was the buzz word du jour or the silver bullet of the month. They were well thought through then, and thousands of "pros" used them extensively - even if they were not Sigma Six approved! So, according to the logic previously stated, all of these "pros" got it wrong, or Mamiya got it wrong? I think not.
My opinion - if a user wants the camera to think for him - buy a digital point and shoot and fill your boots! Heck, one can even get Photoshop Elements to do that thinking too.
However, if a user wants to be a pro and continue to learn and use highly capable tools like an RB67/RZ67/Hassie/Large Format camera to express his/her art, then "quit yer snivlin' and get out there to excite some silver".
Enuff said!