The 3 areas apart from injuries etc. that pose issues for City are sides that (1) are well organized, sit deep & close down relentlessly or those like (2) Klopp who press high intensely & flood the midfield (Like Bristol did briefly) & (3) their lack of central penetration when there are two banks of 4.
And it's true, it takes the right personnel, discipline and bravery to be effective with point 2. I also think with the imminent addition of Laporte, the 3-4-3 will give City the ability to stretch the back line of the defense and force them to make split second decisions on which runner to track as the midfielders, forwards etc will be changing positions constantly while maintaining great width because the fullbacks will really look like wingers allowing the wide forwards to come inside which Raheem & Leroy can do very well
Bear in mind it won't be one set formation but it will change into many over 90 minutes