Been playing dat arm0red core, 120fps game engine lock, w' amd's updated framegen AFMF 2.1 doubler to 240fps, using vsync off (Tp4's outdated TV is only 144hz)
It adds a slight softness, but does increase fluidity.
Seems like a valid use case if your FPS is over 100, and it simply doubles that. Below 100, there's more artifacts.
Realistically, only the extremely fast effects break up a bit, and you can't really tell because they're quick flashes.
For example, Does it matter if a lightning bolt has weird speculars in it? Can't really tell outside of freeze frame.
The rest of the screen, because it's 120fps, doesn't move enough to notice-ably cause artifacting.
As for Variable Refreshrate (freesync), when you double 120fps to 240fps, into 144hz display, the tear is very small, and not easily noticeable. VRR is nice, but if you're over 100, frame gen doubler looks ok.