I tend to use a lot of green bottom canyon pieces as a means of creating scenery for my maps. i usualy clip them into the ground and sometimes into themselves. i often play on a server that has concrete blocks clipping into standard terrain and i seem to have no performance issues. same thing for when i play through my own maps after having rendered the lightning. However, i did have my game crash when i was making a very complex map and im speculating it had something to do with water elevated on heightened terrain, though im not sure.

in case it does not hurt performance that much i had really hoped they made entirely naked canyon blocks without the roads lol.

In my experience, blockmixing hurts performance about as much as using normal, non-mixed blocks. You'll really only run into trouble if you use an absurd amount of them, but even then I find that you're usually okay. One crash shouldn't be anything to worry too much about; that could be an MP problem unrelated to blockmixing entirely. If you're consistently crashing on your complex map, then there might be a problem with them, but as it stands right now it sounds like you're fine.