Interestingly, I heard that one of the biggest stickler points was the fishing details. The UK was more resistant to EU boats fishing in or near their territorial waters. Looks like they were eventually able to get some kind of compromise.
And I honestly had no doubts the border with Ireland would stay open. There is so much at stake there, that both sides probably wanted to ensure that remained stable.
Incidentally, I started this Brexit thread in June of 2016. Hard to believe it is still alive in December of 2020. Guess the talking heads that continued to say Brexit would be easy were a tad off the mark.