Not so much a touchscreen but I've used plenty of stylus/digitizer setups for this purpose over the years and they work as well as you'd expect. At one job, we used normal PCs with simple Wacom tablets for annotation of slides during love Adobe Connect sessions and the only complaint was from some users who hadn't yet become comfortable with looking at a monitor while drawing on a surface (sort of like learning to use a mouse or touch-type for the first time).

At my current job, we have Sympodium monitors at lecture podiums so you can draw directly on the screen. These are also not touch screens but make use of an active digitizer like the Surface products and traditional drawing tablets.

Likewise, I've got a Gen 1 Surface Pro which works just as well for the same sort of thing. Basically, as long as framerate doesn't need to be as high as video, it's fine. Usually the "slides" portion of those conferences (whether web-based or using VTC appliances) is set to use a lower framerate for bandwidth purposes while the "camera" portion will attempt to hit more video-like framerates. But otherwise, it's pretty useful and something of a standard in the educational live conferences I've worked on.

Sort of. I used to use a cheapo Wacom tablet with a painting app for online WebEx demos a few years ago; I'd flip from whatever thing I was demo'ing to a new desktop with a blank raster file open and draw explanatory diagrams. It was actually pretty decent. Having a pen integrated into the device, with some sort of transparent annotation overlay app would have been pretty exciting for me back then (not least because I wouldn't have to hand write text!).

I would think rarely would a normal on the wall whiteboard work well with a webcam on a teleconference, but then again I've never had a meeting room that worked for much of anything teleconference-related. Always something wasn't right, lighting, space, noise etc.