Nexus phones are all about the freedom of buying without any carrier interaction and just popping your SIM card in, and the new Nexus 6P and Nexus 5X are no exception. Both phones have a hefty number of radio bands inside, and while they don't have exactly the same set of radios they both have all the bands you need to bring your phone to any conceivable carrier here in the U.S.
Though Nexus releases have varied a bit on how much the carriers have been involved, this year it doesn't look like there was much interaction between them and Google. None of the U.S. carriers have voiced official support for the phones (nor are they selling them), and while that isn't an issue for the likes of T-Mobile, AT&T or their prepaid subsidiaries (just pop in your SIM and go), it can theoretically cause issues with Verizon and Sprint because both need to have the phone's IMEI in their database to work properly.
The official line from the event presentation and Google's websites is "support for major carriers across the U.S." — and it'd be extremely surprising if there were any hold-ups with getting the phones to work on all of the carriers here. We know the phones have proper hardware to run on all of the carriers, so there's no cause for alarm.
Nexus 6P radio bands
|LTE CA DL||B2-B2, B2-B4, B2-B5, B2-B12, B2-B13, B2-B17, B2-B29, B4-B4, B4-B5, B4-B13, B4-B17, B4-B29, B41-B41|
Nexus 5X radio bands
|LTE CA DL||B2-B2, B2-B4, B2-B5, B2-B12, B2-B13, B2-B17, B2-B29, B4-B4, B4-B5, B4-B7, B4-B12, B4-B13, B4-B17, B4-B29, B41-B41|