Why is there so much uncertainty with SegWit2x when it has 85%+ mining support? I'm confused about the uncertainty and pessimism about the Bitcoin scaling debate. I see coindance https://coin.dance/blocks is signalling a 85% support for SegWit2x. As far as I know SegWit kills 2 birds with one stone i.e. SegWit gets implemented in Bitcoin August 1 and then there is a 2mb hard fork a couple of months later which also has its benefits (and is not as extreme as the 8mb or unlimited proposal).

At 85-90% mining support it looks like the hard fork will have a clear and dominant coin even if other miners do choose to continue mining another chain (+-10%, BU). 

So it may be finally possible to buy that coffee with bitcoin at the end of the year while Bitcoin still keeps all of its current benefits to society. Is there something I'm missing?
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