Except for 2010/2011, I don't think there was any universal agreement about Nani among United fans tbh and his inconsistency was always something to argue. Mane has been getting the boot for his selfishness from Pool fans for some time now, and well, the point you're missing here you can actually get with being erratic if you're a winger but midfield is the position of the most consistent players in the team. Wingers can be inconsistent as they try to dribble a lot but midfield is supposed to provide consistent performance most of time, keeping the ball and moving it from back to front. A midfielder who loses the ball, erratic and chaotic in his output will never help you control possession.
Not sure about Keita as he hasn't been a great success at Liverpool so far imo.