The only part of the brothers’ concerns that is stated in verse 9 is that they desire to be effective missionaries. That very understandable prayer does not necessarily lead into verse 10’s description of the Lord’s response. The Lord tells them to be comforted. Why were concerns over being effective something that led to a need for comfort?
What is not stated is that this is a reasonably small band of armed Nephites entering into their mortal enemy’s homeland. Verse 7 noted that they took weapons “that they might provide food.” They took “swords, and their spears, and their bows, and their arrows, and their slings.” Bows and slings could easily be used to kill game. Spears might, but swords were an unlikely hunting weapon. However, even if those were the only purposes, when Lamanites found them, they would certainly see armed invaders.
Thus there was every reason to be uneasy. Although they were entering to preach, they were entering hostile territory, where they would not be able to predict the outcome. Clearly, comfort was applicable. Their faith allowed them to be comforted, on the Lord’s declaration that they be comforted.