Minecraft wiki entry for Hunger_management, associated text: Disadvantages:
Their availability is dependent on the presence of animals within sight, which can be random and require extensive traveling depending on the biome the player spawns in.
Similarly to beef and pork, at least two sheep must be found to breed them for a reliable supply of mutton.
Salmon cannot be bred, so the player has to hunt for them every time they need food, not to mention salmon are agile and hard to hit.
Since salmon are aquatic mobs, it's almost impossible to kill them with fire to get cooked salmon directly, unless the player has a Fire Aspect sword.