Its better for covering up than the orthodox turtle shell defense because you can slip to your inside to evade punches, however, its worse for countering because you cant see your opponent as well.... Its a preference, but I prefer the turtle because its much easier to throw a quick straight that will interrupt your opponents combinations than it would be in a cross arm, if a guy keeps throwing combos, hes not gonna stop unless you clinch or hit back, and id rather hit back
I personally think it's a disadvantage defensive posture that keeps you limited on counter attacking. Although, it can be effective if the fighter knows what they are doing with it and know when to choose to use it.
Defensively, I find it to be the best guard. Practicality nothing gets through. It does put you in a bad position for offense. When I use it, I plow my way in with the cross arm defense, then I push back the opponent to open up with a few short mauling power shots while they are off balance.