Riot,
Nice, fair and neutral review, cheers.
The unstable water level is a problem many pple faced with external skimmers. Once the return piping gets too long then you get this problem due to the "siphoning" effects caused by the back pressure from the long piping.
For my case, I enlarge the 25mm tubing on the AP850 via a reducer to 30mm then add a Tee before the water flows back to the sump. With this I get very stable water level in the skimmer.
You'll noticed that the deltec skimmers come with a clear pipe with an elbow, that is meant to be used with the slotted pipe together with the blue sponge for micro bubble removal (that is part of the installation kit), the slots in the pipe will break the siphon effects. With that you do not need replace the elbow with the Tee.