So apparently UHD Blu-rays are extremely sensitive about having a perfectly clean disc. I opened a brand new movie last night from the shrinkwrap, put it in the player, and started watching it. At about 2 hours into the movie playback froze. Upon ejecting the disc and examining it closely with a flashlight I found a small spec of debris on it. Something several times smaller than a grain of sand or salt. I blew it off with a puff of breath, put the disc back in the player, and playback resumed without issue.
I had previously seen some forum chatter about this otherwise I would have never guessed that such a tiny spec would halt playback and cause the issue. I certainly will not be buying any used UHD BDs that I can't inspect in person.