Each entry contains explanations for why they are in a given category.
Fairphone due to having some online code activation thing for OEM unlocking. Bootloader of my FP4 has been unlocked for quite some time now, but if I recall correctly, this activation step was required for me to load custom ROM on it.
https://support.fairphone.com/hc/en-us/articles/10492476238865-How-to-unlock-and-re-lock-the-bootloader
https://shop.fairphone.com/bootloader-unlocking-code-for-fairphone
This website disappears, you can't unlock your bootloader.
That is why Fairphone is in Caution section.

Microsoft being in "safe" is a bit funny, and the explanation acknowledges that as well.
But it is there due to it containing only two Surface devices, which have easy and standalone bootloader unlock process.