I have a late 2014 iMac that I went from Sierra to Mojave. I have a Benq 27" connected with DisplayPort and have had no problems at all with it. I too did a clean install and it went quite smoothly. I did however have a few minor software hitches but these likely are not the OS fault.
I figured I'd give it an evaluation first before I took the total commitment to the internal drive, so I formatted an SSD with APFS and then made two partitions. This APFS is really quite cool in the APFS formats the whole drive and then the partitions on the drive are sort of floating in you don't define a certain size, rather they can just virtually exist on the drive. My idea was to split off the users data from OS partition onto it's own. To move my user data, I did a CC of the users directory from my Sierra install over to the newly created User-Data partition. I created my user account then just changed the location of the home directory. Worked like a charm except for a few apps bombed. It turns out its a bug inside their code and not that I did it wrong. Also, I use Firefox and for some reason, it lost access to all my plugins. Easily fixed.
Quite liking the feel of Mojave. Working with the app developers to get there software fixed.