It's not about security, it is about privacy. While MicroG in theory could bypass NetGuard, I very much doubt that anyone would bother. My privacy is not that precious.
But as I said, neither solution is great. How about sandboxing MicroG too?
Again the only advantage of MicroG compared to play services is that it's open source, you still have all the same privacy and security issues.
Its already a lot of work to support the official play services and make them work in a sandbox, supporting another layer in between is more headache than its worth it or they have time/money for. Not to mention that sandboxed play services work with much more feature than MicroG such as android auto.
...assuming you are connected all the time, or at least that the services are running all the time. In my case they are not. I only enable them every once in a while, when I need to be alerted of something. This might not be how most people use their phones, but I do it because there is no way to preserve any privacy at all if you are running Google services 24/7 (sandboxed or not).
I see Google services as malicious software - not security malicious (Google can't risk that) but privacy malicious. This is why I care more about ability to turn them on / off than about what kind of access they have. Even inside the sandbox, as regular apps, they have way too much info about me.
As I said: I would prefer sandoxed MicroG, but given the available options non-sandboxed MicroG is preferable to sandboxed always-on Google services.