Perhaps slightly off-topic but I was wondering if half the problem with unsupported rails comes not from the rail itself, but from the commonly used clamping method to hold it?

I can imagine those rather immaterial aluminium screw clamps allow a fair bit of movement and flex under use. On the other hand, if the rails precisely slot into a much more solid piece of metal and are directly threaded and screwed up tight, I bet the movement is reduced a lot. This is how the CNC3040 and 6040 do it.

I'm guessing most who have used unsupported rail in DIY machines have gone the clamping route, because the cost of end-machining the rail to have a thread and perfect flat end makes it easier to buy supported rails.

Supported rail will be better, of course, but I'm just wondering where the main area of weakness might be.