After updating to Ubuntu 18.10 today I noticed that a lot of games were either crashing to the desktop, not detecting my Steam Controller at all or detecting it as a second player.
With the Linux Kernel version 4.18, a driver is now included for the Steam Controller named "hid_steam" which seems to currently be conflicting with how Steam and plenty of games handle the Steam Controller resulting in behaviour we don't want.
For now, it seems you will need to blacklist that "hid_steam" module from loading if you want to continue using the Steam Controller as normal.
You can test it yourself, using this terminal command to see if it fixes it:
sudo modprobe -r hid_steam
That was only temporary though, a reboot quickly made it break once again (but it works for now). To have it permanently stay, you can do this:
sudo bash -c "echo 'blacklist hid_steam' > /etc/modprobe.d/sc.conf"
I did also run this command so the above was correctly detected:
sudo update-initramfs -u
Once you reboot, that will then make it stick.
Hopefully this situation will get sorted out, because that's really not ideal right now. For newer users especially, it's not a good look. I've opened a bug report with the Ubuntu team.
Update: As noted in our comments, there is a Kernel patch available if you wish to patch it manually yourself if you're confident doing that sort of thing. Hopefully the Ubuntu maintainers can pull it in for everyone.