We praise Ammon as a great missionary and rarely mention his brethren in the same light. We take Ammon's success and often assume it was because he was more perceptive in how he approached missionary work - by serving rather than preaching. However, it's worth considering something as it relates to our assumptions and natural biases.
Each missionary in question fulfilled his mission obediently, but Ammon got the easier assignment, frankly. He got a king who offered him a daughter to marry BEFORE he started serving the king; his brothers were rejected out of hand and ended up being abused in prison. I'm not about to criticize them for their work and tell them they should have just done what Ammon did - that they would have been more "successful" if they had understood missionary work better and/or had chosen to serve instead of preach. That's like saying those who get called to
Likewise, why do we measure missionary success as a function of numerical baptisms - especially when the "missionary manual" (Preach My Gospel) says over and over again that we should not do so?