The only disadvantage is you lose control over the browser version that's running, but you also lose that with a shared runtime (unless every app pins their own runtime version which defeats the purpose of sharing). Besides, most Electron apps also have a web version that has to support all browsers anyway. The advantage is you seriously reduce the disk and memory footprint of your app and shed the responsibility of shipping prompt patches for Chromium security issues.
and a Node.JS runtime, and native libraries to support menubars, system trays, dialogs that support folder selections, native drag and drop for all file types, ...
I agree, though: if the electron app doesn't need any of these things, it really shouldn't pull in the entire electron runtime.
And we're back in Java-land, but with HTML instead of Swing :-)
It's nice from a developer point of view but I wonder how many application creators/vendors will actually use it. Most want to control as much of the stack as possible.
Idea of runtime mode - https://github.com/electron/electron/issues/673