Can't say for ASM but Octopus uses a sliding tube to set the water column height. This makes it difficult, if not impossible, to fine tune the skimmer. I assume that ASM may have similar problems.
The benefit is adjustment of the foam head, there is a "sweet" spot in every skimmer this just makes it easier to find and maintain, skimmers that use one pump for bubbles and flow are more tricky to set than dual pump models
