I can't even reliably install Windows 10 on it. I managed to successfully install it once, but I had to reinstall (see reason below) and now it always freezes during installation (either when "Copying Windows files" or "Getting files ready for installation").

It has got serious boot issues. For some reason it wouldn't even power on until I unplugged it from the mains and let all the capacitors and whatnot fully drain.

The DP->HDMI adapter outright blows. No matter what I did (updating motherboard's BIOS to latest version, updating LSPCON to latest firmware via Intel's site, and installing the latest graphics adapter drivers), on every damn system restart there's a black screen. After some time I can manage to at least move the mouse, but there's still no proper image (i.e. I can't see the Windows login screen).

Considering all the issues, I suspect I have a faulty one. I'd really like to RMA it, but the vendor I bought it at has gone bankrupt, and since there's iirc only a 1 year warranty on it, the warranty has expired since I bought it late in december of 2016. I can't believe how much money and time I've sunk into this. I mean it's almost 2018, can motherboards just work? Does ASRock actually have an internal QA program and test their designs before they manufacture, and then test before they ship? If they do, then not very thorough. Next time I'll wait for the reviews on products before ever dipping my toe into anything ASRock.

And maybe use the DVI output and a lowres/fullhd screen for the install.

Which W10 are you using, latest ISO download?

My J3345-ITX has been very stable (except UHD60 via HDMI2).