Tried it in a few older games.
Resident Evil 4 (2005) looks extremely smooth at 120fps. I recommend turning off the motion blur effect in game as it looks a bit weird with the interpolation (and isn't needed now anyway at 120).
Jet Set Radio, which has a 30fps lock on PC, didn't fare well. There is a lot of artifacting around the edge of the screen when in motion, and around HUD elements when panning the camera, maybe due to a combination of the simple art style and going from a 30fps base framerate. I would honestly just recommend playing at 30.
Metal Gear Solid V looks great at 120fps and the advantage here is not having to use any mods to go above 60fps, which I believe can break some game logic.
Katamari Damacy Reroll is an interesting use case for the software. The rendering of the gameplay is locked at 30fps but the overall image has an arbitrary framerate it seems, so you will catch certain UI elements and some scenes running in a high framerate. This causes an ugly mismatch which LSFG actually works well to makes less obvious. Still noticeable, but less so.