Eh?..3-5-2 is a preeminently defensive system..teams that deploy it often concede the fewest goals. What's also true, and you quite rightly pointed out, is that it gives you numbers to flood the midfield and to press the opposition higher up the field.
The WBs in a 3-5-2 also have shorter distances to cover and play a more intelligent role rather than having to constantly get up and down the full length of the pitch.
The CBs have to be more versatile though, particularly the flanking ones as they have to cover more ground and must also have the ability to initiate the first phase of play ie. they have to be better vertical passers. I can see Joe Gomez excelling in this role.
3-5-2 is a more defensive system for more defensive teams because, in the static defensive position, it basically reverts to a 5-3-2 or a 5-4-1. For teams that spend a lot of time on defence, that is basically what it boils down to, as it nullifies the game on the flanks against the opposition. The wingbacks for these teams generally spend more time closer to the defensive line than they would in a more advanced position against stronger teams, which would be for the majority of the time for teams in the bottom half.
But for teams that play a high line (us), 3-5-2 (or a 3-4-3 for that matter) allows for a better lateral game in midfield, closing of the diamonds, allowing for the outball to the flanks in order to cycle possession and avoid pressure. But it will leave gaps in behind for the vertical ball, and if your CB's don't play compact enough, it will allow for runners in the half spaces to receive the ball from the flanks (like the goal Walters scored against us). Therefore, a crucial element is that the CB unit must be able to learn not only to move as a unit vertically, but laterally also in a more compact manner. If one is pulled out of position on an opposition counter, the remaining two are then left to deal with most likely the main opposition striker, and traditionally this will most likely be an aerial threat rather than the ball on the ground.
Then you have two options. One is to have a world class goalkeeper who not only excels at shot stopping, but is also confident in commanding the situation (i.e coming out on a consistent basis to deal with the aerial threat). But to be honest, those don't exactly grow on trees, and Migs is definitely not one of those. The second option is then to have CB's who are more able to be physically strong enough to deal with the strikers (or trailing runners for that matter). If they are not strong enough or are good enough aerially against this threat, you will often see teams score against the run of play on opportunistic pops from the side.
