If you know exactly how the oil will behave, a fixed head works fine. One wrong guess = boom.
The purpose of any system is to keep the head submerged at all times, they just vary by method. Engines that can have a pump drive extension (SBC) can put the pump right in the oil.
Reason #1 for swinging failure: the pivot seals were loose, worn.
Reason #2: the sweep of head positions didn't cover the entire oil path. Some appear to have little foresight: pivot is almost directly above the head, with a short arm. This only works if the oil remains in the sump (which is the swept path).
Obviously, the best pivot is parallel to the oil level, and the swing-arm is very long so the intake always presents normally, but this isn't possible with some blocks, cranks, pan designs.

BTW, another advantage to dry sump: you can have as many individual pickup heads as you have scavenge modules (gear-set + container), since exposing one of them has no effect on the others. Design ooppsss is far safer.