Regular chocks require a hard, flat, non-slippery surface to be effective. They don’t work well on soft grassy fields, muddy areas, or on icy spots. They are not effective on tires that are raised on leveling blocks. To be fully effective, they must be used in pairs because they impede tire rotation in only one direction. They can be used on any single tire regardless if it’s on a single or double axle trailer.
X-chocks, however, can be used only on tandem/double axle trailers because they rely on the counter rotation of the adjoining tires to lock the tires. They don’t rely on friction with the supporting surface so they can be used on soft grassy fields, muddy areas, and can be used on tires that are raised on leveling blocks. They work better on tandem axle tires that are close together and not so good on wide spaced tires like those trailers that have a load “equalizer” brackets between axles.
Just my 2-cents.