Minecraft wiki entry for Hunger_management, associated text: Disadvantages
Melon seeds can only be found in structure chests (mineshafts, dungeons, and woodland mansions).
Melon blocks are only found in jungles, savanna villages and woodland mansions.
Melon stems grow slowly and don't instantly spawn melon blocks even when grown with bone meal.
Only restore 1.2 saturation per slice, so a diet of melon slices needs frequent stops to eat.