I think it’s valid for boosting raw 60fps to 120fps (or whatever your max refresh rate is) in games that aren’t super latency sensitive. Boosting something like 180 or 240FPS to 360 or 480 is probably valid for more latency sensitive things like competitive shooters since it would (theoretically) increase readability during motion at an imperceptible cost to latency. I’d probably use multi-frame-gen for boosting 120->480 if I had a super high refresh rate monitor so long as the system has very good native frames to start from (which is… Arguable in a ton of modern titles).

Using it to boost, like, 30->60, or even 45->90 is hellish, as the latency at native 30 is already annoying and at 45 is just barely pushing into the range where latency is less noticeable, so adding latency is just a really bad trade.