Cristiano Ronaldo is just abysmal at crossing, if you watch him enough you will know he rarely delivers the kind of balls that hang in the air for the striker to head in, and usually loop into the box which is what defenders and especially keepers relish dealing with. Also with Ronaldo you'll get him attempting shots from 40 yards out every single match, and rarely ever releasing the ball early, instead choosing to delve into his bag of tricks which quite frankly have gone stale and predictable.
There is a good reason why he's been dropped onto the bench in recent times despite being the undisputed first choice on the right wing for a year or so.