The main feature of the new model is that it has less functionality than the old model. Firefox is betting that it can enable close to 100% of the functionality that users actually want via clean APIs instead of opening up literally every internal aspect of the browser to extensions. There will be bugs, but specifying APIs makes them easier to fix.

The advantages are speed, security, ease of installation, ease of development since more thought was put into the APIs, compatibility with Chrome extensions, and better compatibility across platforms since extensions now use less native code. Those benefits are "observable". The downside is that you have to wait for Firefox devs to think though the security, performance, and usability aspects of every bit of functionality an extension dev wants, and then wait for the new or updated extension to exist.