Uncomfortable on the ball is largely down to how much players want to retain posession and are comfortable with the ball - so yeah its largely related to them being under pressure, so a defender who has that trait is likely to just lump the ball away when put under pressure because he doesn't have the confidence/ability to do otherwise ... the technique is important because it can cause a player to be under pressure more easily, ie. I'm an amateur defender, when I (cough) control the ball from a pass often that means it ends up a couple of feet away from me, this gives opposition players more opportunity to close me down than a more technical player who might have it instantly under his control etc.
(does that make sense?)