I just bought the 4k Plus due to AV1 codec support. I moved from the gen 1 4k stick which still works well.
For anyone that does buy this 4k Plus or Max stick I will save you a lot of grief with either streams freezing or juddery motion...
Ensure that you select Dolby Digital or PCM rather than "Best available ". Also do not enable dialogue enhancer as that will also cause issues with the streams freezing.
Finally (and most importantly for me) if you experience juddery motion on certain streams/channels (usually 25 fps) even with frame rate matching - set colour to RGB on your firestick. There are some users on the internet having this issue but no one found the solution, or at least reported it. It took me a few days of testing to figure all this out. I hope it helps someone.
EDIT: 25FPS is very temperamental on these new firesticks due to Android 11 issue I think. My firestick 4k 1st gen plays 25fps streams perfectly but on the new 4k plus it stutters. At times it works fine and then the next time it's stuttery as hell. 24FPS, 50FPS and 60FPS are fine. I'm thinking of returning it.