Thales had a system outage for a few hours on Sunday. We lost service mid-flight on our first leg and wrote it up. The Thales specialist (well, he was a mechanic wearing a Thales hat) told us of the outage and verified the aircraft's system was working properly. The green check mark was present, which apparently indicates a connection to the satellite, but no data would get through. We continued on a second leg, in the same airplane, and an hour, or so, into the flight the internet returned.

The OP listed the 737, A319 and A320. That covers all three internet systems used on UA mainline aircraft; Thales, Viasat, and Panasonic. Of the three, Viasat is the newest, and by far, the most reliable. Viasat is on all 737 MAX aircraft, a few 737-900 aircraft and will be on all of the narrowbody aircraft that received the refreshed United Next interiors.

The UX fleet uses the ground-based GoGo network.

You can see your aircraft's network provider on the portal page when browsing to unitedwifi.com on the aircraft's wifi network.