ULD's are not "sealed".
Imagine a (ULD-shaped) box, now remove the front of that box and replace it with a fabric curtain.
Given that (even when it's secured) it's effectively "flapping in the breeze", IMHO, if there was a pressurised container in the ULD that "let go", the excess pressure would firstly blow the front flap around, and secondly rip the front flap off, long before there would be any structural damage to the container.
SpannerTwister