With no windings on the rotor, they are not subjected to centrifugal forces. And because the windings are supported by the housing, the rotor can be cooled by the conduction media, do not need airflow inside the motor for cooling. All of this means that the motor’s internals could be entirely enclosed and protected from dirt or other foreign matter.
The stator is potting by epoxy, and separated from rotor by isolation layer. So the waterproof performance is very reliable. This kind of pump is amphibious and maintenance-free.