Many of the Ryzens, like the 5600, are power efficient enough that they'll not actually hit socket limits under stock conditions, but still hit max boost. So for those cpus, PBO basically does nada, but offer up a 200MHz addition to the boost, and maybe then they'll hit the socket limits. The 5900x isn't one of those. With a load like Prime95, it'll hit its 142w limit, and put a hold on the boost. Enabling PBO will raise those limits, add the 200MHz and essentially set the cpu free to boost its heart out.
That's when ppl run into heat issues on budget or undersized coolers. The 5900x has enough cores and radiated heat under PBO to exceed the cooler capacity, resulting in high temps, and lowered boosts. Undervolting can help that somewhat, but done too excessively it hurts single thread performance too much, which drops fps on the master thread, even though temps and voltages seem right, and they are, for multi thread scores.
It's a balancing act really, to determine what temps and voltages get best overall performance, regardless of what the user thinks they shoukd/could be. It's a Ryzen after all, not an intel cpu.