That's a good question, actually. There are various strategies for converting framerates, some of which work better than others depending on the ratio of the rates involved. With modern equipment, this sort of thing is a lot less of an arse than it would have been historically, when pull-down would be more practical than interpolation[1].
I suppose one complicating factor is that often the old film is sourced in some intermediate format (eg. modern 24/25/30FPS colour film, analogue video) and the artefacts of the conversion process remain.
Obviously if the camera wasn't recording at a consistent framerate (eg. hand-cranked, or clockwork/battery running down) and there's no time reference to correct it, it's always going to be a bit of a fudge.
Oh, and there's a circle of hell for documentary makers deliberately add artefacts to old footage in order to emphasise its age.
[1] Notable counter example: The video from Apollo 11. That was 10FPS SSTV, crudely converted at the time to NTSC by the camera-pointed-at-a-long-persistence-CRT method, hence all the motion blur. I'm sure most of the time we see that footage it's a copy of the live broadcast (presumably via 30fps film, amongst other things), not derived from the un-converted NASA tapes. (Indeed, there was plenty of high-quality colour film shot during the moon landings, but we hardly ever see that on the telly.)