I bought the Pioneer 8400NEX last August, and had it installed in my 2017 WRX with the Maestro RR adapter to allow continued use of Starlink, steering wheel controls, factory backup camera, and adds OBD-II gauges.
It's been fantastic so far, and it's nice that I don't always have to dig out my iPhone from my pocket or backpack. I have not tested battery drain while keeping the iPhone connected and unplugged, as I like to keep my iPhone on a magnetic Qi vent charger/stand. But I have no problem making calls wirelessly, using Siri to read my messages, controlling spotify and Tidal music, and finding my way with maps.
I only have a few gripes with Wireless CarPlay vs my previous 8200NEX using wired Carplay.
(1) With the 8400 only one of the two USB ports can be used with CarPlay and iPod Video, and the other USB port is only for using with Android Auto or a thumb drive. So, you can still have your iPhone connected with CarPlay and your iPod Video connected with USB, but you cannot connect two iPods or iPhones to a USB at the same time.
This means that when I'm driving with my son in the car who wants to play his music, and I don't want to disconnect from CarPlay to let him take over (then I can't use hands free calling and texting), then we have to dig out my iPod 5.5G from my center console, unplug it, connect a 30-pin to lightning adapter, and let him plug his 7+ into that.
It's easy to switch our iPhones as the source on Wireless Carplay, but when he's connected to CarPlay to play music and I get a phone call, the he has to pause the music, answer the call on my phone, and put my call on speaker. So, instead we dig out the iPod to swap the cable to his iPhone.
I sometimes use a 60GB 5.5G iPod via USB because when using a 128SD SD Card (in the slot behind the screen) or a thumb drive via USB, I have to rename ALL of the songs on the drive to have a song number in front of the song name, but I don't have to do this when using an iPod for music.
(2) Wireless CarPlay uses WiFi and it disconnects my iPhone from my home WiFi when I start the car in my garage. The problem is that if I am on a phone call using AT&T WiFi-Calling then I lose the call when I lose WiFi (We only have 1 bar LTE here at home, with iPhone 7, 6s, X, and Xs Max, so WiFi calling is a must).
(3) When I am using CarPlay with Apple Maps, and I want to change the music or need to dictate a text, my map screen goes away and the iPhone doesn't display the map either, as it also switches to whatever app is on the CarPlay screen. Likewise, if my passenger grabs my phone to open the browser or read an email to me, the map disappears from the CarPlay screen.
Apple should make the MAP app appear on the iPhone when the CarPlay screen is displaying the music or other screen. So for long trips, I have taken to using my spare iPhone X for maps on the dash vent, which is better than the built in Pioneer navigation, while using my primary Xs Max for everything else and CarPlay which I don't need to keep up on the dash vent at all.
(4) Occasional crashes - not as often as my 8200 though. Almost 1x a week out of 20-30 drives I will have the 8400 crash or the Maestro RR will say it's not connected, requiring me to shut down the car and wait 10 seconds. powering off the Pioneer doesn't seem to make a difference. It's more like only 3x a month.
(5) I don't think the 8400NEX with Wireless CarPlay plays nice when the iPhone is also paired via Bluetooth. It was causing issues with not being able to access CarPlay, and I had to unpair my iPhone from the BT. This has also happened in my Daughter's Jeep Compass with wired CarPlay built in, so...