[Philbo King]

AFAICT it doesn't make any difference if video items align to a frame boundary.

I don't know about how it handles interlaced video. Is that even used any more?

I see video get resampled to the project settings when imported (dragged in) from Media Explorer. You can tell if that happened; it is indicated by a green "I" above the imported video media item. If you prefer to avoid resampling, before importing use Project Settings to set the audio sample rate to 48KHz and video resolution and frames/sec to match the stuff you want to import.

Video crossfade artifacts during playback are pretty common, especially if there is a lot of on-the-fly resampling, effect plugins, and system resources are tight. I've found (so far) that rendered output always works correctly though.

When you render, note the render speed ratio. If it is less than one, it means your system can't play this project back perfectly in real time due to CPU/RAM/drive bandwidth limitations. You can freeze tracks with effects on them to relieve some processing load on the system. You can also reduce resolution in Project Settings (instead of 1920x1080, divide both mumbers by 4) so playback uses less resources (You can still render at full resolution).

I'm still a beginner at video in Reaper - I've edited 10 or 20 perfomance videos and put them on Youtube/Facebook. I recently got more ambitious; I'm recording video of my tracking sessions for a new song and importing them all into the Reaper audio project to make a music video. All this is to say there are probably things I got wrong in this post; I'm counting on other forum members to straighten me out on those things if I made any mistakes.