I was getting around 40-50fps with RT and all that jazz in the AC Shadows benchmark (game was free, I've never spent a dime on any AC game).  Playing with fluctuating FPS that low sucks so I locked it to 40fps (120hz monitor, gsync).  Fluctuating FPS between say 80-120 is totally fine and smooth.
I played for about 4 or 5 hours at 40hz and it just looked so terrible.  The motion clarity is garbage.  I'm team green but FSR frame gen can be enabled along with DLSS in AC Shadows.  I unlocked my framerate and turned on frame gen.  Wow, motion clarity sky rocketed, beautiful game.  And honest to God I did not notice any input delay which is insane.  Most areas were over 100fps and it felt great (was playing with a wired controller so, kb/m might have been noticeable).
40hz is definitely slightly better than 30 but honestly it's not a big difference at all.  You're kidding yourself if you say otherwise.
I see a lot of folks here get excited and praising a 40hz option.  Pointless topic I know, but FUCK 40HZ, it sucks dick just like 30.
Have a great Saturday