Bear with me, I don't own one anymore but I've owned one personally and several in my small company. I sold my personal unit to a longtime friend, she returned it to me several weeks ago to deal with a death - my point is that she didn't use it much, and it was meant to replace her old Mac laptop that runs Tiger and that she would appropriate the 2013 unit when her life settled down. The 2013 unit is a late-2013 2.6 i7 with 16GB RAM and the 1TB SSD, and it had HS installed (still does, for that matter...). Sooo...
I installed a new clean Mojave Volume at Beta 9 and now it's running the latest Beta update - I have a Dev account - and I figured I'd try it out before she got the prompt to update and had to learn about a new OS just after learning a new OS, if you get my meaning? I'm her IT person, but I have a company to run - one OS a year is enough for her...
Cutting to it, and having a 2012 i7 Mini Server and base iMP to compare the rMBP to, IMO Apple's done a pretty masterful job with Mojave and it's very stable on all 3 Macs. This past week, updated drivers for all of the peripherals that the rMBP will be attached to (wired and wireless) have been issued and installed (mouse, Wacom Intuos Pro tablet - she was still using an ADB ArtZ II!, and two printers) and it's running very well. No issues, no freezes, no KPs.
I can't comment on whether to clean install or update. APFS is a bit of a different beast, but on all 3 Macs at hand I installed a clean Volume. The rMBP will have a clean install when I hand it back to my friend, likely within the next 7-10 days. I liked Sierra on the rMBP, I prefer Mojave on the rMBP now - but it's not my full-time Mac now. FWIW, I did buy my rMBP new so I've had plenty of experience with it. I won't install the GM on my iMP or Mini Server, opting to wait until I get certified builds of some of my software (Autodesk and Nemetschek apps) but I'll be installing the GM on the rMBP and be confident that it'll work just fine.
The main issue I've had with Mojave has been with touch and stylus drivers, and they were all updated on Friday...
