Seems to me (and that doesn’t mean much) to be the most secure / well updated option. GrapheneOS on a Pixel runs GREAT for me, I honestly expected a buggy experience from a custom ROM.

Also, my grandmother could install it (this was especially enticing, I was worried about having to flash an OS, didn’t want to brick a phone).

You get a lot of flexibility when it comes to installing Google Play Services. What I do is install any app that needs gplay services to a separate user profile with them installed.

I honestly don’t have any cons, I’m completely satisfied with it, I can’t see myself switching back to an iOS device or trying stock Android.

I can think of two cons, although they don’t bother me (not sure if this applies to every ROM available):

You miss out on some features / apps that come with stock android, such as AI features.

AOSP apps are installed out of the box, but aren’t wonderful for day-to-day use IMO. I recommend Fossify and You Apps

https://www.kuketz-blog.de/grapheneos-der-goldstandard-unter-den-android-roms-custom-roms-teil7/ This author made a series about custom roms (unfortunately only in German, but Firefox has a built in translation function) where he tested what data was sent in the first 24 hours of using the custom roms without installing any new apps or changing the settings. To summarize it: Most custom roms still build up a connection to some Google server (like a SUPL server for example) and some custom roms have rather bad security practices (like waiting for a month before rolling out security updates). GrapheneOS is the only custom rom that doesn’t connect to any google servers and has exceptional security (even better than Stock Android).