Well, strictly anecdata but there was a time when I worked on code which for $reasons had no dedicated test environment, and externalities were not our problem according to management. So we were reaching out to the big bad world for testing, but we didn't own the network between it and us. I ended up writing tests for the network. Caught a lot of problems with the network. Didn't make any friends.

DISCLAIMER: I've been around IT for probably the majority of y'all's lifetimes, so I'm not saying this happens often. But just because something is fundamentally wrong, doesn't mean that all fundamentally wrong things are the same. In my experience they differ more from each other than the possible good ways of doing the same thing. Don't conflate things without a good reason.

Oh no, a well-written unit test is 100% deterministic.

It's once you start doing tests in real environments with real databases and real networks that the unpredictability of the real world creeps in!