Unless they've changed it since I last looked, the biggest problem is you can only completely remap the function of the buttons 100% of the time, so you can't just have flappy paddles in synchro mode and normal F/R shifting in manual mode. I've never had a problem with the buttons with gloves though.
(Also, if you've never connected the app before and it offers to do a firmware update, there's a non-zero chance it'll fail and brick your bike, so don't let it go ahead if you have an important ride tomorrow...)
I never got the hang of synchro but then I never persevered with it, the problem being I tend not to back off as much as I would for a rear shift as a front and not being too sure when it's going to shift the front...
If you have the SC-MT800 Junction A, it'll do a double beep on the shift before the double shift. The standard road Junction A doesn't seem to have a beeper.