I'm jumping into this conversation with the hope that others can comment as well. One downside to these kinds of applications is the ability to save a live presentation. For example, when preparing a flipped lecture, one will want to draw on multiple slides and record a video of the lecture in real time. That is pretty easy to do now.
The challenge comes in if I wanted to save the presentation as a conventional slide deck with my annotations in place. Powerpoint on the Mac allows for slide annotation, but only while in presentation mode, which creates a mismatch with screen size between a large iMac and the iPad Pro (adjustable in Astropad, I know) but even worse is the pen tool in powerpoint is far too pixilated and just plain ugly.
Sadly, Keynote has not caught up with this functionality yet. Text looks much better when using something like Screenink over Keynote, but I'm back to the same problem of not having true in slide annotation. I think Keynote in iOS does allow for in slide annotation, so we almost need a reverse application of Astropad where the iPad screen is projected on the iMac where the video is running.