Starting with the galaxy S2 and now the current S9 I've never used it. After I read the post I thought give it a try couldn't hurt. Learned something new.
Imagine going through the new phone setup process with a display brightnes of 30%. Add to that a 3 second screen timeout and a white keyboard with gray text and tiny little keys. I have to lean under a table lamp and type. When finished the first thing I do is turn it off and get to the play store to turn off auto update.
Auto Brightness being slow to respond is on purpose (at least in my opinion), it results smoother brightness adjustments when the phone is exposed to rapid ambient light changes
As for the brightness going way up during unlock, it might be a way to grab some additional detail for the hybrid Face+Iris unlock if you're using that. Else, I have no idea.
I think you're right about the brightness increasing when using the Intelligent Scan / Face Recognition for the unlock. When I turn everything off except the fingerpreint scanner, the brightness doesn't increase. When I turn on intelligent scan, it start's doing it again.
In the facial recognition settings under lock screen and security there is an option that brightens the screen when you unlock you phone. Just turn it off and you wont have to turn on your phone to a really bright screen.
I had the same problem with my S8 and figured it was just my phone being bugged... after upgrading to the S10 I realized it was a setting in the face unlock biometrics which can be disabled if you'd rather enter a pin or fingerprint to unlock your phone when it is dark out. I never really use auto brightness so it was odd and annoying my phone did that to me