I've never been convinced by Windows Fast Startup, even with a Windows only installation. There's a lot of talk about the improved performance, but actually ALL this means is that (if it's working) your PC will start faster. On older machines this may make some difference, but with a recent device with a fast cpu and SSD, we're talking about a handful of seconds. For me, the main issues are that it isn't 100% compatible with automatic updates. It certainly isn't with some of the 3rd party programs I use, and it isn't even 100% compatible with Windows' own update process (which is absurd). I have my main Windows PC turn off at night after its backup finishes (it used to run 24/7 but the criminal conspiracy to keep power prices absurdly high means I look for savings now) and another machine that runs 24/7 turns it on again in the morning. With Fast Startup I had problems at least once a week, often to do with updates, but sometimes with various servers, eg VNC, not starting properly. Disable Fast Startup and this goes away. As you found, Fast Startup also locks the system drive which royally fucks any multi-boot setups.

The boot isn't noticably faster without. It didn't really fuck up the multiboot per se, though I was unable to mount the ntfs volumes in linux without rebooting into Windows 10 first. What I hate is that when you choose Windows 7 from the win 10 boot menu, it reboots and then loads windows 7, instead of just proceeding to load it. Anyway that it works at all is a minor miracle. So far I'm liking Windows 10 pretty well.