Just got my Pixel 2 yesterday and got it activated. The “sweet spot” appears to be immediately above the fingerprint sensor under the glass part. I’m finding the read performance is about the same as my old LG G4. As usual, you need to direct skin contact, and you need to learn the best phone orientation to read your tag. I have not tried a case yet. I AM planning on getting one where I may have some trouble reading the tag, but we’ll see, and I’ll reply back.
Now, I AM somewhat disappointed with the latest Oreo updates (though I hear this may have come earlier) where they disabled 1) NFC Smart Unlock Trusted Devices (i.e. no more NFC unlock) and the NFC on/off home screen tile (swipe down).