Constant water level means constant water level, not a 1/2" flucuation, or 1/8" flucuation; a constant water level. The only way to do that is with a baffle. No matter how cleverly you think the ATO is designed, the water level will flucuate, and the more the ATO cycles the sooner it will fail... Also LR has no place or function in a sump. It is a complete waste, doing just the same thing as bio-balls of days gone by— anyone want to fill their sump with bio-balls? I have NEVER run across a system that was suffering from so little rock, that more was needed in the sump...Many have so much rock in the DT that it inhibits good circulation....
Wow I don't think I have ever read anyone get so excited over such a simple statement before. Thank you for bringing your opinion into focus and it will be noted and entered into the record for future reference. However and I know that you know this with all of your years of experience and being a pro, but there are multiple ways to accomplish the same end result. That is one of the great "secrets" of this hobby. You agree? Remember in the old days when the internet wasn't around and you had to problem solve on your own? For example: When I built my system I put baffles in to control the water levels and to compartmentalize everything but I bought a new larger skimmer and it just did not fit. What should I do? I don't have the money to go buy a new sump because I just spent the money on the skimmer. Think, think, think. What should I do? I know, because I am moving the system to another part of the house, and I have to drain the sump anyways, I will cut the baffles out and TRY it this way. If it doesn't work I will just move the baffles over a few inches and reattach the glass. As you can see below my skimmer worked just fine in a sump without baffles and I will never build a sump with them again (or until I change my mind).

According to your train of thought it should not work though it makes you wonder how much better it would if I had it in its own compartment. The water level in the sump was much higher than recommended by the skimmer manufacture so how did I overcome this? I built a platform out of pvc pipe and eggcrate to the correct water depth. Those dreaded microbubbles? I know, I will block them from my return pump by turning the inlet away from the skimmer "section" and put LR between the skimmer output and the pump return so the bubbles hit the rock and float to the surface. I suppose I could have taken large diameter pvc pipe, cut it in half and blocked the pump. Well look at that, once again multiple ways of doing the same thing. See the trend?
As far as your statement for LR being the same as bioballs you realize that doesn't make much sense because they are not the same because they don't function the same. Yes both are used for biological filtration but there are differences but I am sure you already knew that. One completes the process and one does not. But we are talking about one system right? The display tank is connected to the sump and fuge and ..... so why is it if you have a system with LR and BBs you can get nutrient readings? Every time someone would debate this I would think about it but it wasn't until RHF on here talked about the rate of breakdown due to the physical distances between the bacteria.
If you really believe that having LR in the sump is bad you had better get busy and start posting in people's threads warning them of their mistakes including some TOTM reefers. Some have LR in their sumps and others do not. Once again there are multiple ways of achieving the same goal.
Finally as far as the ATO failing I have never had one fail after years of use. I am sure people have because anything mechanical can and will fail at some point. If you are using something like an aqualifter pump to top off you can see it slow down over time. Often the lines get clogged especially if you are using Mrs. Wage's pickling lime (oh once again, another way of doing something) but cleaning it with vinegar is pretty easy. If that doesn't work, $15 will buy a new one.
Perhaps you have tried this and failed or have had poor results but others have not so you shouldn't dismiss this so quickly.
I suppose I wrote too much for today's standards for posting so if you did not read any of what I wrote I think this sums up my points:
That's one thing I love about this hobby, so many different techniques work.
Thanks.