Water cooling, even preassembled aio's have their quirks. There are more parts, more moving things like the pump etc that can fail. When a pump fails it means buying a new aio vs merely replacing a fan on a heatsink. Aio's normally don't leak but it's not impossible either. Every now and then someone has one leak and best case scenario the pc just shuts down due to thermal throttling of the cpu. Otherwise if it leaks it typically leaks on the gpu or other components and can take them out.
Whether it looks better or not is personal taste, neither is wrong. I tend to prefer air coolers, not only for safety and quiet cooling but for appearance. Others prefer the look of water cooling. It depends on the cpu, something like an fx 9590 may very well require a dual/triple fan aio. An i5 or i7, medium to large air coolers are plenty and overclocks will typically top out due to vcore restraints/limits rather than thermal issues.
Custom water cooling is where liquid cooling really shines for heat dissipation, looks and quiet operation (lower speed fans on an overradded system). They're pretty expensive and a lot more work, maintenance etc but they also use much higher quality pumps and things.