On UA, it’s definitely variable.

On 3 of our last 6 UA flights, wifi only working on half of them. To be fair, one flight might have just been bad seating choice - we were in row 18 on the 753 and noticed I had an unstable to non-existent connection when seated, but once I was about the cabin it seemed to work fine. Wife was the same. I suspect we were in a particularly weak area between routers and so our phones just weren’t connecting to either.

While in general in-air wifi has improved, still don’t consider it to be too reliable - I’m of the mind to plan to be without it.