Definitely a size limitation with this design but, thinking about it, that might be a positive with very small parts that would be hard to hold otherwise.
Very much agree that ANY movement of the piece is a deal breaker - whether due to the board shifting, runners not snug, friction, milling of the board (and likely other factors). I was trying to hold the test board firmly when trying to dial in my sled (flat - not like the one in the video) but was apparently not 100% as my cuts improved just by putting some 200 grit sandpaper on the back fence face (the kind that comes in a roll with a bit of adhesive on the back).
Bevels are a challenge so appreciate everyone's thoughts and suggestions.