Like I said, simpler is better for ATO and gravity feed is about as simple as it gets. The only real failure scenario is the mechanical float valve. With any kind of electrical set up you're adding more failure modes. Every pump, electric float switch, controller etc. can each fail and even a power outage will put it out of commission. The trick is to size your reservoir reasonably so that it will keep your system topped up for about a week (or however long you need when on vacation) so in the event of float valve failure (open) you won't have a major flood or change in water paramaters. The other failure mode of course is that the float valve gets plugged & doesn't top up. I run a variation of the gravity feed with an air pump on a timer pressurizing a glass container to push the water through a mechanical float to my display since I don't have a sump and not feasible to have elevated reservoir. If I could I would. My improvised system has worked trouble free for well over two years.
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