petermc H265 offers better compression than h264, which basically just means you can get higher quality files with less data. H265 also has a larger maximum resolution that's playable with today's hardware vs h264.
So in general, h265 is just straight up better in every way. Slightly more difficult to encode, but better.
A far as why you're getting jittery picture with some h264 scenes, couple of possibilities I can think of. Depending on your GPU, it may have difficulty playing larger h264 files. Like I mentioned before, h265 files are more efficient, and often h264 scenes try to make up for it by utilizing a crazy high bitrate, which some GPU's (aka older ones) struggle to handle.
Could also honestly be the studio you're watching, if you've noticed a pattern with the scenes. Some studios who use h264 may be using an older cam or not have the hardware/time/effort to re-encode scenes to h265. I've noticed a few of the smaller studios who tend to have a bit jittery of scenes, or scenes that looks like they're 30fps instead of 60fps.
