All in One (AiO) coolers are closed loop (usually). There's nothing to fill up but its also not expandable. If one part breaks the whole thing is done.
Their performance in relation to cost is indeed not that great. A top level air cooler will cost less while doing a similar good job AND providing some air flow over the VRMs in some way or another. They might (!) get a little (!) bit louder but that's about it. I agree with @SolarNova, if they offer top of the line performance they come at about twice or thrice the cost of a high end air cooler and way worse performance compared to even a small entry level custom loop. Compare let's say a NZXT Kraken X62 with the EK Fluid Gaming 240 set. Yes, radiators ar of different sizes but you can bet the custom loop performs better for like $50 more. Granted, the aluminium parts aren't particularly easy to expand outside of EK's Fluid Gaming lineup.
Concerning leaks: there's hardly anything that could leak on AiOs. They're not meant to be disassembled so there are less potential leaks compared to a custom loop.