Anecdotal:
Had a plenum up and running for 3 years on a 55. Never had any signs of problems that I could correlate to plenum use. Because of a move, I had to tear it down and was expecting to find some nasty, sulfur compounds below the plenum. I was surprised to find no stench and really no black sludge. I also had a large refugium and adequate skimmer associated with the system and a fairly deep (5-7"), mixed particle size substrate (CaCO3) on top. I could grow the heck out of softies and SPS alike (I did drip kalk as my auto topoff)
So, who knows what contributed to the success of the system, probably all of the above along with consistent water changes. To be honest, I think on a macroscopic level, over a 3-4 year period, it would be hard to differentiate success between many of the husbandry styles utilized. In other words, they all work as long as you recognize and compensate for their limitations.
A couple of pesos...
Take er easy
Scott T.