Thus far, if I had to voice a complaint about Android Auto, it would be the same complaint I have about Android Wear: No true support for Google Voice numbers when doing SMS texting. No matter what you do, you can't initiate a text or reply to one with your Google Voice phone number. It insists on using your phone's carrier-assigned number (which I don't ever use for any of my communication). Phone calls go out through my Google Voice number just fine; but not texts.
I haven't tested Hangouts instant messages yet. I suspect those should work fine (as they do with Android Wear). Of course, with Android Wear, you still can't initiate a Hangouts instant message (you can only reply to one). As I said, I have yet to test this on Android Auto, but I suspect it's the same situation.
I've provided feedback to Google (through the Feedback option in the Android Auto app), requesting real support for Google Voice phone numbers; but who knows if they will ever listen and do something about it. As it stands, replying to texts with Android Auto is not a good solution for me (a Google Voice user).
On another note, pertaining to these Pioneer head units, I have noticed that while they have the ability to pair to more than one phone at a time, they cannot dynamically switch between them for Bluetooth calls. In other words, if I hook up my personal phone to Android Auto (which still uses Bluetooth for calls), and also pair my work mobile to the head unit, if a call comes in on my work phone, I still can't answer it via Bluetooth. I have to choose which phone is the "active" Bluetooth device on the head unit. Only one can be "active" (i.e. able to make/receive calls). I literally have to go into a menu and choose which phone I want to use for calls during my drive. So if you use two phones, forget about using both of them on the Pioneer units.
My Jabra Freeway Bluetooth car speaker can maintain two phones and will allow both of them to make/receive calls, on the fly. Once both phones are paired, I don't have to select anything or make any changes to settings on the unit. So, it looks like I'm going to keep my Jabra Freeway on its visor and use it for my work mobile, while I keep my personal phone attached to my head unit for Android Auto. That way, I can still keep both phones hands-free. That's a shame, considering the price of these head units.
P.S. Checking voicemail with Android Auto's "Voicemail" option in the dialer is also a no-go for Google Voice users. It dials your carrier number (which doesn't contain your Google Voice voicemails), so it's worthless.