Running poweroff
is one of the correct ways on anything Systemd (details). If that doesn’t work then something is broken.
If you haven’t done so already try looking into the journal. sudo journalctl -b -1 -e
will take you to the end of the log for the last boot.
Try launching Steam from the terminal so you have a chance at seeing an actual error message, at least for the crashing games.
It might be the kernel as the other comment says since the 9070 is pretty new. If it works without issues on something like Fedora or OpenSUSE TW then that was probably the issue.