PGAS is not an "alternative hardware-accelerated message passing mechanism", unless you use a definition of message passing that is so expansive that the statement becomes vacuously true. It's distributed shared memory, integrated in the memory hierarchy, which you can manipulate at the same granularity as other memory, which you can have pointers to, etc.

You can have, say, a 100,000 x 100,000 matrix represented as an array over thousands of processors, where each processor can read and write each array element individually.