i have 2 new UNO Wifi Rev2.
When setting the Serial baudrate to 115200 i get a lot of times garbage sent by the board.
Max seems to be 57600.
(On my regular UNO this is not happening.)
This is quite a limitation.
rolf
That does seem to be a limitation of the mEDBG chip. 
Man... It's a crying shame how they saddled the awesome new chip with lousy design decisions like that... Like, just stick a generic serial adapter chip on it, load it with optiboot, and it'll work fine!
shrug I guess I'll be selling 4809 and 4808 "pro mini" style boards (ie, stick external adapter on it) soon enough... maybe then there will actually be a usable 4809/4808 board available for arduino users!