Why is there so much uncertainty with SegWit2x when it has 85%+ mining support?
The problem is that this will most likely result in a hard fork. 

There is no well tested core client which supports segwit2x. The bitcoin core developers oppose that. Confusingly, their proposal (just called segwit) announces a soft-fork which is designed to be compatible. 

In the end, this is a political game. Miners tend to support segwit2x, while bitcoin core developers support UASF. Their statement about the danger of a chain fork can be found here: https://bitcoin.org/en/alert/2017-07-12-potential-split