Developer of the app here, happy to answer any questions.
replies(26):
I wish I had the time to port it to all three desktop OSes.
If you don't care about making the best possible app that you can, go ahead and do it in the browser. You will get something that's probably good enough and runs everywhere. But it's going to use more battery, more memory, and more bandwidth and not feel like a Mac app. Plus (IMHO) it's less fun to develop for the browser.
It’s possible to make great web apps, it just takes the kind of care and dedication that @milen has already proven to have. If the web interface lowers the barriers to developing a cross-platform version of Monodraw, then I think it would be silly not to consider investing in it.