They are quite memory expensive and take more GPU time to render than the simpler LODs, so it will be a matter of deciding for each usage, they’re not too bad though - it’s definitely possible to create a whole world with 100s of 1000s of trees with OI LODs running to a good framerate.
I like your idea about masked areas for collision - I’ll have to give it some thought - but yes, it would be nice to have a system that could just create a simple cylinder collision for the trunk and nothing for the leaves… I don’t know how far you could take it though - I’ll do some experimenting once I finish rdLODtools 1.4.