For ‘Reliable’ read ‘Meaningful’.
Here's a discussion on the values it can return, Mesh.publish() successful but returns false even though gateway receives message - #7 by kubark42.
I personally would love an API which includes an ACK for a specific receiver. This way a mesh could have sleepy elements which only wake up long enough to ensure their messages is delivered to the gateway, without considering if the gateway is currently able to publish the message online.
It's trivial to build an ACK of this form, but since there is already a mechanism to ensure that a gateway gets (and forwards to the cloud) the mesh's Particle.publish() command, it would be nice if this API could be exposed to Mesh.publish() as well.