HexHyte

I sent them a message regarding the dumbing down of the sensitivity configuration. If enough people complain they'll do something about it: https://embark-studios.helpshift.com/hc/en/3-the-finals/faq/71-how-to-contact-us/

What is wrong with this game... why they had to do this instead of implementing a user-friendly way to insert our favourite value with accurate precision directly ingame...
 

Now that the game has been officially released, there's a new option for mouse sensitivity called 'Focal Length Sensitivity Scaling' that you can toggle on and off.
I believe it indicates the ability to adapt aiming speed based on the sight zoom, which might solve the issue of having to choose between prioritizing scoped sniping or iron sights.

Are you following the instructions in the notes? If you add any decimals it will revert to default settings:
Before you make any changes to the config file, you should first change the sensitivity in-game to a random value that is not 0, 100, or the default 50. Make sure this change causes the sensitivity value in the config file to have six decimal places. This is important because adding new decimal places will cause the file to become corrupt, which would cause it to reset when the game is launched.