How secure is File Vault 2 on Mac, given the fact that resetting/finding out someone's login password is not impossible?
File Vault won't protect you from someone with the means to grab your laptop when you have it unlocked and image it. Neither will it prevent someone from coercing your password from you or just snooping it from reuse / yellow post it notes, etc... https://xkcd.com/538/



FileVault is designed to prevent trivial surreptitious attacks:


booting to single user mode to add a new admin account
looking at your data (or copying unencrypted files) by booting to another OS that someone brings to your Mac


Filevault is good protection from people that can't or won't steal your password to unlock your drive. In a nutshell, it enforces authorization to decrypt your data, apps and OS with a password of your choosing. 


https://support.apple.com/en-us/HT204837