Well, using Sidequest is safe. Or as safe as any mod or beta test programs can ever be. So it should be fine. At worst, you might need to uninstall and reinstall Beat Saber due to the game file getting borked somehow.
Lo, a quest! I seek the threads of my future in the seeds of the past.
Once you get comfortable with the process to re-install after each Beat Saber update, it is completely awesome. Read the instructions carefully, it is easy to make mistakes. I also use Side Quest's back-up function to restore my scores after each update. That involves figuring out where various files are on the Quest so you can restore them.
And when there is an update, make sure that BMBF has a compatible version out. Occasionally you need to wait to install the Beat Saber update until BMBF has updated.
This is my advice if you already have bmbf......do not upgrade to the multiplayer until all the bugs are fix......if you have the one and 2...... upgrade the 2 only....for now.......i can't get my custom song from bmbf on multiplayer.....the package song is fine but not necessarily what I like......