This reminds me I needed to reply to a few of the above points:
but I've come to a point where nitrate and phosphate are steadily climbing as the scrubber simply can't cope.
If you are feeding the same, and N and P are rising, then your scrubber is weaker for some reason. It just can't get weaker over time if the flow and light are kept up. If this idea is accepted, then you'd need to look at changes in your tank that are trapping more waste. This happens to me in the eel pipe (waste cannot get out, at all), and it used to happen in the sump when I had chaeto in there (it would trap waste). But even though I feed the eel a silverside once a week (which all wastes in the pipe, like a dead fish), and even though I feed liquid food (pure phosphate), with enough scrubbing power the N and P are zero.
but if possible that will be substituted by cheato or caulerpa to save on lighting costs.
This is often mis-understood, that chaeto etc can "do the same" with less light. Think about the chemistry: One photon of light goes into one cell of chlorophyll, causing one molecule each of N and P to removed from the water. Another photon of light causes another molecule of N and P to be removed. Thus, you cannot remove the same amount of N and P by using less photons of light. The amount of N and P removed is proportional to the amount of light. What you can do it try to get more light using less electricity, which is a different topic.
I could build a huge scrubber with massive lighting, and I'm sure that would cope with my increased bio-load, but it will simply cost too much to run.
But remember that if you are feeding the same, and if your scrubber is kept up, then you are just trapping waste somewhere and the particles are not getting to the corals. Instead the waste particles are rotting in the water; and while the bacteria that feed on it also feeds the corals directly, the N and P generated are much more than if the waste were just eaten directly by the corals. Good circulation in the tank helps kick up waste for this purpose. Three of my five powerheads point straight down behind the rocks to do this, and the remaining two are pointed to create a circular flow in the display.
I believe that the more means of filtration you have at your disposal, the better. With the skimmer, scrubber, DSB, live-rock and substrate in my system, I'll be able to sleep peacefully knowing that the system is well able to deal with any eventuality, rather than constantly running on a knife edge as I have been with only the scrubber.
Well your sand are rock are not going anywere, so if you mean redundancy, that's what separate scrubbers are for. Separate screens, pumps and lights (on separate circuits). If you mean protected from a bad event (like a big dead fish which everyone likes to use as an example), remember that a skimmer does nothing to help in this situation at all. Nothing. If a fish is going to die and kill things because of the ammonia, then a skimmer will not help at all. A scrubber however will start absorbing the ammonia right away.
At some stage you'll have curve-balls thrown at you, such as cyno blocking light on your screens
There should be no cyano in a scrubber at all. If there is, the flow is WAY too low, probably the result of a clogged pump. Mine got this way after one year of not cleaning the pump. The slow-down is so slow that you don't notice it. It went from one-inch of pooled water at the bottom, to it being almost dry.
or algae not sticking to the screen properly anymore
This can't happen, any more that a skimmer can all-of-a-sudden not make bubbles. Screens don't go smooth by themselves overnight. Actually they never should go smooth. How could that happyen, anyway?
or power-failures
If you set up a scrubber properly, with the pump and the light on the same circuit, if the pump goes out, the light goes out too. It can sit there that way for longer than your tank will survive without power.
You don't want to rely on only your scrubber coping with a high bio-load when this happens
A scrubber (algae) is the best way to deal with high bio-loads. I'm assuming you mean ammonia/ammonium, which is what bio-loads produce. If your rock and sand can't handle a large ammonia increase, then algae will. Your skimmer won't do anything, at all.
I enjoyed the challenge and novelty of running a system in as natural a fashion as possible, but I enjoy catching fish and raising them into fat happy chappies more.
All the more reason to give the small fishes more plankton to eat, without skimming it out.
1) High electrical consumption.
One CFL watt per gallon for high filtering, plus the pump.
2) Algae death as a result of a power failure eliminates your filtration for at least 2 weeks.
Power failures do not kill algae. A full screen can sit for 6+ hours with no real problems. The water collected on the inside layers keep the outside layers from drying out. And acrylic (enlosed) units can go much longer. Your tank will be dead first. A newly-cleaned screen can't sit for quite as long, but it doesn't matter because it was just cleaned and therefore was not doing any filtering anyway (the filtering would be coming from the other un-cleaned screens in an alternate-cleaning setup. And certainly, if your tank is worthy of worrying, it's worthy of cleaning only half of the screens at one time.)
3) Limited bio-load potential. (The speed at which algae grows even with optimum conditions has a limit.)
How many new fish are you going to add in a week? A blank screen can reach full capacity in a week. However the other fully-grown screens in your alternate-cleaning setup are fully available to absorb any ammonia/ammonium immediately, which is what you are trying to absorb. A skimmer, however, has zero bio-load potential... it does not remove any ammonia/ammonium/nitrite at all. So with skimmer or without skimmer, you still need something to handle the "bio load". Algae does it instantly (in minutes), via currently absorbing ammonia/ammonium, and it does it long-term, by growing more volume.
4) Fluctuations in bio-load are not easily absorbed.
The opposite. Fluctuations in ammonia/ammonium/nitrite, which is what you mean by bio load, are instantly absorbed by algae. That's what algae do, even when the scrubber lights are off. And since you would alway have algae in your scrubber (because you only clean one half at a time), even in darkness, the algae would instantly, directly, and completely abosorb the ammonia/ammonium/nitrite down to the level that the algae could not absorb anymore (which is an un-measureable amount).
5) There is zero tolerance for low maintenance.
Multi-screen systems, and especially acrylic systems, are very tolerant. If you let one screen go uncleaned in a multi screen system, the other screens absorb what the un-cleaned one doesn't. And new tests with acrylic units are showing that they can go much longer than 7 days between cleanings, because the algae cannot get "thicker" and block the light, since the acrylic keeps the algae close to the screen. And the higher growth rate of most acrylic designs compensates for the pods.
6) Extreme general increase in humidity.
Totally eliminated with enclosed designs. Considering that you can have an acrylic box built for only $100 USD, I'm surprised that you did not do this considering how much you have invested in your tank.
In my opinion, adding a protein skimmer is the only solution.
I think the electricy is the only thing that a scrubber has as a negative. But remember that a skimmer does not remove nutrients, at all, so you still have to pay for something to remove them (especially phosphate) in addition to running the skimmer. And if your rock and sand can't handle the nitrate, which obviously they can't because you say it it rising, then you have to pay for something else to remove it (in addition to the skimmer). And this is all on top of the skimmer removing most of the food that all the corals, and some small fish, need.