Compare this to something like Qt, which is open source. But (as I understand it) it's LGPL licensed, meaning it can't be used commercially without paying the developer for a commercial license or releasing all the source code.
Of course, take this advice with a grain of salt. I've not really worked with Qt or Ultralight before, but I've been exploring my options for a project for a school club.
[1]: https://discuss.ultralig.ht/d/6-ultralight-0-9-beta-now-avai...
Edit: added source
That means you can dynamically link QT, and your program doesn't have to be (L)GPL. No commercial license required.
That's nice, but not being open source means it will never have the breadth of players who both use it and contribute back that Electron does, which is why it won't beat Electron on the long term, even if it had some intriguing advantages.
Edit: When I say open source, I didn't mean it has to be free of charge.