I have a refugium that sits below the tank level. Mine is working out fine. In my situation it just wasn't possible to sit above the tank. I have to agree that a lot of pods
probably do not make it like this, but there's still a lot who do. The explosion of the pod population in my main tank confirms that. For me, the main purpose was moreless growing macroalgae and outcompeting problem algaes, specifically turf algae. In my case, it has done just that.
I still think having it above the tank is the best way to have one. Unfortunately for me, it just wasn't an option.