Ppl liquid cool just the cpu all the time. That's the entire thought process behind an AIO to begin with. Setting up a custom loop that does the same job is just more effective as the pump is stronger, flow is better, and tubing is tailored to what you specify, not what the AIO comes with. Another advantage of custom loop is you can add to it at your discretion, add a gpu or ram, or if available, even motherboard cooling.
But everyone who uses an AIO liquid cools just the cpu, unless they also spring for an adapter and use an AIO as well on the gpu (nzxt makes some, as does AMD).