I did an upgrade from chimaera on a system76 laptop and it went flawlessly.  It's a pretty generic install and I mostly use it when I travel.
Today I tried to do my server/main desktop, ASR motherboard with i7-4790K, 32 GB ram and GeForce GTX 1660 graphics card.  There's a lot going on there, so I don't want to risk an upgrade.  I prefer to do a bare install so I have notes on everything I need to install anywhere.  I of course use a separate partition so I can boot both and take a few days or a week to upgrade different pieces like virtualbox.

After doing the install, the system did not boot, which is common for me with this system.  I guess I should just tell it not to install grub.  My UEFI bios was able to be set to the right thing to boot and I got things straightened out.  However, I then did not get a graphical interface, just a console.  startx just silently failed.  The log showed a core dump due to illegal memory access and a backtrace which meant nothing to me; the nouveau driver was of course being used.  (I didn't immediately save the log and it seems to be overwritten now.)

Rather than asking for help with nouveau, I added contrib and non-free to my apt sources from the console, and installed the Nvidia driver, which I ordinarily use anyway since nouveau cannot handle 60 fps 4K videos.  All is well now, but I thought I'd report this in case anyone else experiences it.  Nouveau used to work, just not fast.
--Tim