well I have a motherboard with UEFI firmware ; i'm running Slackware current , with secure boot off. My simple concept of UEFI is that it was to prevent PC's from being booted up with malicious software. Thats means someone has to have access to my laptop in order to do that and UEFI will still not prevent a PC getting rootkit or virus once booted up(although so far I never had a rootkit or virus) so its a pretty narrow area of concern to me. With more than one OS being used if one of them has GRUB then, GRUB can be used to boot other OS's. I'm not sure with how that would play with MAC though