For reference, the Slack OSX App uses MacGap.
I don't think this kind of technology is inherently bad, and not everything needs to have a perfect native interface. But I don't think people should be mislead into thinking that this will allow you to make apps that feel just like truly native Mac apps.
If you already have a great web app and want to allow users to install locally instead of having to keep a browser tab open macgap is fantastic. You can do a minimal amount of work to add great functionality and you dont need to hire a bunch of objective-C devs to clone your web app.
Maybe it could be better with a performant view layer like React, but I'm not sure. In the end, native apps win.