I had ultegra di2 6870 on my main road bike. And in the winter I found it a pain with thick winter gloves. And had some battery issues.
Ive changed it to 9100 mechanical and find it much better.
As for shift speed imo my older record 10 speed group shifts quicker than anything else I’ve had inc Di2 but never done any proper testing.
If you think synchro shift is for you then go for it.
On my mtb the xt di2 has been faultless, it’s 1×11 with a shift button by each grip. Right side for down left for up