An AIO is an air cooler with added water to leak and pump to break. It will always be more likely to break down and have much, much higher potential impact when it does. The up side is it can have a larger radiator than is practical for a normal air cooler as it can be mounted further away and the weight taken by the case. This can in some scenarios result in better temperatures although the tradeoff is being slightly noisier - though in some scenarios the fan noise may be lower by enough to make an overall quieter machine despite the pump noise.
Are they terribly unreliable things that burst all the time? No. But they are less reliable and the benefits are fairly marginal in many cases - they are mostly a fashion/trend item.
I agree, but differences aren't marginal when they allow for any ram to be used without any kind of clearance issue. I'll go aio next for that reason alone most likely. They'd sell far more air coolers if it wasn't for this issue together with featuring customisable rgb.