@Pen3: You can hug a shield close to your body and it'll still serve its purpose (blocking incoming blows)
Whereas if the enemy get too close, the SCAR can't easily just aim at their weak spots because it'll hit the other body parts instead.
If you go look at page 5, you can see it's as long/longer than his arm length, whereas the P90 even with its silencer added is probably close to 2/3rd or half that length.