@Louis Dabout one thing you are forgetting. A competent scutum user has a nigh impregnable defense a longsword wielder cannot penetrate.
@Sir Rabby how in the hell was your elbow sticking out? Unless Stannis has mutant long arms a scutum or tower shield is 1.5 times the width of the user's shoulders. So unless Stannis has abnormally long arms or the shield was made for a smaller man Stannis losing his arm is bullshit. Along with chopping through the metal shield.
In cannon it took a dozen full powered swings of a Valerian steel bastard sword on the same spot to penetrate steel plate. Even the the blade was nicked, cracked and damn neat broke. A gladius doing the same with less blows. Blows landing on different areas of the shield at that, it is impossible for Stannis to have cleaved the shield. He does not have the Mountain's strength nor the leverage to do it.