shared_ptr means reference counting, reference counting means you lose determinism because you no longer know if releasing a reference will trigger releasing a resource. Delay and offload releasing the resource to a separate thread, you lose your guarantees when a resource is actually freed, too, just like using a garbage collector.

And I know it for the .NET GC, they tried a reference counting GC as alternative to a collecting GC and it performed worse and comes with the cycle trouble.