Any form of aerial shield pressure that involves landing near the opponent requires some sort of lag canceling (auto-cancel, L-cancel, float cancel, etc) in order to be effective. For example, Falcon's strong knee -> jab cannot be shield-grabbed when done properly, as well as Fox/Falco's NAir -> shine or DAir -> shine. If you don't L-cancel, you run the risk of getting shield-grabbed without proper spacing. That being said, truly un-punishable shield pressure with spacies is something that requires frame-perfect precision, considering that players have to time when the aerial hits the shield (so they can fastfall), the shield stun, when they can input the fastfall, and when they can input the L-cancel. This reliance on perfection is something that players like M2K exploit; in
this match, for example, you can see how M2K does a slight light shield and angles his shield slightly upward. This changes the amount of shield stun the players undergo and when the shield stun occurs, causes Lucky to miss one fastfall in the middle of his NAir -> shine pillar and get grabbed. While it's hard to tell whether Lucky got punished for missing an L-cancel as well as the fast-fall, the point is that L-canceling is a technique that requires precision, and this precision can (in some cases) be exploited by the person being attacked.