Definitely this, unfortunately. elementaryOS 0.4.1 Loki was fairly stable for me, but then I installed 5 / Juno, and now the machine often locks up with a completely black screen, except for a handful of items visible where the top menu bar usually is. It seems to occur when it wakes from a sleep mode (though this is on a Parallels virtual machine). The only solution is to reboot and lose whatever you were working on.

elementaryOS is great and the first Linux I've really enjoyed using (I'm normally a Mac user, the kind who uses software by Panic). But it still has so far to go. It's also worrying that you can't upgrade from 0.4.1 to 5. Even with all of Microsoft's disasters with updates, they've still managed to let you upgrade from one version of Windows to the next one.