Main drawbacks that I read on this forum:
- Google Pay not working (but you can use your iPhone for Apple Payor a smartwatch)
- Some banking apps not working (there's a list and a pinned post about app compatibility)
- Preinstalled apps look out of date (but you can install whatever apps you want)
- Proprietary Google features do not all work like find my phone, cloud backup etc. (but often there are private alternatives)
Other than that, I see only advantages. Especially security and privacy wise, you will improve a lot with the many features you can see on the official website.
Android Auto works now btw., also pretty much every app depending on Google Play Services which will now run in a sandbox without access to all your data and behaviour on the phone.
Your iPhone is a very pretty, controlled and secure environment, but far from privacy respecting (especially towards Apple themselves). With GOS, you'll get advanced security, privacy and agency over your data.
The good news for you are you already have a Pixel and an iPhone. You can try GOS out wihout risk as you can always go back to Pixel OS and restore a backup there. And if anything doesn't work at first, you could use your iPhone for an intermediate quick solution, although I doubt it'll be necessary.