I have no actual complaints about the Pixel to be honest, and I'm usually hyper-critical of consumer electronics.
I do have some criticism of Android the operating system, but I'm not sure this is relevant to Pixel per se. My complaints are all things that affect nearly all manufacturers (Samsung, Xiaomi, LG, etc).
I also have lots of criticism for various individual apps (looking at you, YouTube Music), and google assistant (please stop making breaking changes to the voice "command line arguments").
But as far as an android physical+firmware phone goes, the 4a is damn close to perfect. Add hot-swap batteries and dual SIM (hey 5a!) and it would actually be perfect for me.
Honestly at this point it probably sounds like I'm a marketer who posted a weak criticism of Pixel and then responded with glowing praise using an alt-account but...this is my experience anyways.