What advantages does Elementary have over (its upstream) Ubuntu? It just looks like a different GTK/GNOME Shell theme to me.
Disadvantages: * Tons of bugs. A lot of bugs have not been fixed since a long time. So, if you find a bug, be prepared to fix it yourself. * eOS devs also make their own Code editor, Mail app, music app, videos app, photos app, calendar app. Some might say that this is a waste of time. * No minimize button. And this is a little frustrating. The devs want you to use the workflow they recommend to the point that they went through the effort to hide the key and make it so that you can't enable it.
Gnome Shell on 18.04 pretty much killed Ubuntu for me. Next time I set up my computer, I'm going back to Elementary. (I used it some years ago, but it was still very buggy. I'm hoping they've fixed some of those since.)