To put it simply:

- If you run even a single app with integrated Google Play libraries, running sandboxed GSF won't enable any more telemetry capability from Google.

- MicroG's spoofing approach means it will likely brake at some point and nuke your whole setup.

- Running MicroG as a privileged system app increases attack surface.

- Spoofing also means less app compatibility.

- MicroG still runs proprietary Google code.