That is true. Ideally, you'll have the libero behind a back four where the fullbacks doesnt bomb forward and you wont have anyone in the DMC area and NO playmakers. That way the libero will get involved ALOT, but somehow i cant make it click overall, so I did a compromise. A compromise that I think works great. Some say the Libero role is broken, i dont think it is. You just need to think and observe the movement of the other players to build a team where the role works.
Same goes for other roles. Take this tactic..at first I wanted the left wingback to stay wide to make room for the false9 and the CML and Libero, but the wingback role worked better in this tactic when he combines in narrow spaces with those 3 players. Guess thats what makes FM so addictive...that you have to watch and think.
And yes. PPMs can be vital. Runs with ball through the middle can be a fun one too for the Libero...if he can dribble

Switch ball to opposite flank is useful as well.