A big one for me is choice of apps. Only apps made for Android Auto are available, and that's not a lot. Especially pretty much nothing from F-Droid. Open source apps for Android Auto are very limited.

Sideloading is not a thing for Android Auto either. So I can't even choose to work around the limitations as I can on the phone itself.

The ancient Android Assistant sucks. It constantly doesn't understand what I ask it and so far I haven't figured out when exactly it can do a google search for me and when not. Especially considering how far LLMs have come in terms of answering simple questions (e.g. if a name comes up in a podcast and I want to know a little bit of background information to that person). And when they finally add LLM support it will be Gemini only with no choice past that.

Then there's stupid design decisions in apps, e.g. that Google Maps doesn't show GPS speed when in Android Auto mode. I also hate that Google Maps decided in an update a year or so ago that speed camera warnings (which are illegal in some of the countries I frequently drive in) can not only not be disabled but are so important that they need to take up half the screen and hide the navigation directions while they are up. This is especially crappy when I come up to a busy highway intersection where I have to get off and suddenly there's no navigation on my screen but a stupid warning I don't care about because I obey speedlimits anyway.

And lastly (that's likely down to the implementation in my car or my phone), there's constant connection issues.