how accurate is an ATO system?

Why spend a lot of money on an ATO when you can easily make them yourself? Much cheaper and works just as well! Your best bet would be to go on forums to see how other people are making theirs. You can make gravity powered ATOs that don't rely on float switches and batteries, or you can make ones with float switches, which are accurate too, but a little more difficult to put together and might require a little soldering or electrical knowledge. Float switches will probably be more accurate because you put the float switch at the level you want the water at. This way, when the water falls below that line, the switch turns a pump on and fills it back up to that level. The problem I have with float switch ATOs is that if it malfunctions, it will just dump ATO water into your tank. I have included links below for DIY ATO systems.

The problem with anything "automatic" is that something can always go wrong though.