CMs in the 3412 are more reluctant to defend than the CDMs in the 352. In a 352 you can launch an attack just as effectively with the right instructions, but the defence is more solid.
They're great because you can play really narrow if you want but have that wide support if the box gets packed, I love 3-5-2
I find for me it's the most effective formation but quite difficult to get right- the LM and RM really have to be able to defend and attack very well which can make it difficult getting players to fit the formation