I'm not a frontend mobile developer, so definitely not entirely qualified to comment, but it doesn't open in Safari (MacOS or iPadOS). That said, I did try uploading Watson and Crick's paper on DNA and it seems to upload but won't open (hangs on 0%) in both MacOS Firefox and MacOS Chrome (both downloaded fresh today). I also tried opening the MacOS app on a computer with MDM controls and it was blocked (need a valid signature on your dmg).
Ginger Labs (publisher of Notability) has dozens of employees and has been in operation for 14 years, so it's a bit unfair to compare, but I definitely like the idea of have a Google Docs style wide margin to play in. Something like MathPad for LaTeX would be nice as well. Lots of asks, I know.