First of all, most encoders do not handle VFR+VST well or at all.
VFR (Variable Frame Rate) When a video switches from one frame rate to another on at least 1 frame.
VST (Variable Scan Type) When a video switches from Interlaced to Progressive, or Progressive to Interlaced, on at least 1 frame.
I personally tested ffmpeg, Handbrake, Adobe Media Encoder (AME), x264, and x265.
Most will more-or-less just leave it either interlaced, or encode to an averaged frame rate constantly.
Basically, unusable and rubbish results.
It's quite common to assume a DVD is either Interlaced or Progressive when in reality it can be both, and is actually quite often so. MediaInfo only states Progressive or Interlaced as it chooses whichever occurs for most of the video.
For example the USA Region 1 NTSC DVD for Family Guy is VFR+VST all throughout the entire each disc of Volume 1 all the way up to at least Volume 4, probably others too. S01E01 is mostly progressive so MediaInfo states it as Progressive (not VST, but does state it as VFR). That's bad as you would not look into deinterlacing and will end up with hard-combing or mice-teeth artifacts, aka baked-in interlacing.
You're probably thinking what are all those programs I tested failing at doing? They fail to recognize when a video changes it's frame rate, and/or its scan type (whether its progressive or interlaced). They will (like MediaInfo) take whichever is most common in the source and then assume its like that for the entire video. This means, for example with Family Guy, it will encode the entire video as if it was progressive (so it skips deinterlacing entirely).
And guess what? That's exactly what happened to Family Guy Volume 1-4. At some point during editing they were seemingly working with mixed-scan material. This resulted in Adobe's software hard-combing a fair amount of frames. You can see this in S01E01 where there's a large group drinking cans of beer in front of the TV (near the middle-end of the episode). This even caused that scene to actually be shorter than it was meant to. This happened so early during editing and production that Audio Voice-Over work was done on the baked shorter version so even if you manually fix it, or re-animate it, etc. it will not be able to sync up correctly.
All of that caused because of incompentancy in what is meant to be professional software. But it's also something a lot of people ignore or just don't know about, hence this document.