The only issues with Shimano are the constant changes in front mech/ shifter compatabilities over the years. You just dont know what you're gonna get when mixing it up.
Sure, down tube/ barcon shifters help with friction on the left but when using brifters, it can be a minefield.
The rear end has been pretty stable for the past 30 years with even 10spd mechs compatible with 7/8/9spd shifting.