A good point was made above, and I want to reinforce it. If you mount a switch to anything, screw it down, because the ballast will work under it eventually.
If you want to let it float, that works too, that’s what I do, but make the backer board about 1 foot longer than the switch(s) so that the connecting track does not try to lift the switch. Your ballast should be consistent in size, what will happen is that you will get about one “layer” of ballast under the switch.
I prefer to float my track, but either method will work.
For drainage, if you float it, washing out of ballast will be minimal, since you have one layer underneath. If you fix the track to the backer board, I would drill a few drainage holes.
The nice thing about using the backer board is (besides it being and STAYING flat) is that you can level the switch(s) by levelling the board, not pulling or pushing on the switch itself.
Any real wood will warp or expand over time.
The synthetic wood will not warp, but it is about half as rigid as regular wood, and will sag over time unless supported more often (check the sites that make this “wood” for decking, they recommend supporting it TWICE as often as real wood.) Think of the stuff like particle board, we have all seen particle board shelves sag over time. The difference is that real wood has nice long fibers, the synthetic stuff is plastic or plastic with sawdust in it. No reinforcing fibers.
Regards, Greg