The current salads (Shroom, Fruit, Veggie) give 6 hunger points or 3 chicken shanks. This seems kind of inefficient, considering that the ingredients used in the salads (with the exception of the Shroom Salad) combined are worth more than 6 hunger points.

 

Berries give 1 hunger point, apples give 4 hunger points, and watermelon gives 2 hunger points. All in all that's 7 hunger points, whereas the fruit salad only gives 6 hunger points. This makes fruit salads only really useful for condensing a lot of fruit into smaller spaces (though you'd need a lot of bowls) at the expense of hunger points.

 

Veggie salads on the other hand remove the Weakness effect of eating wild carrots, so that's an actual benefit. However, until fruit salads are either more efficient or given special properties, eating fruit separately is preferable.

They have a chance of giving you a good buff.  Fruit Salad gives Haste, Veggie Salad gives Health Boost, and Shroom Salad gives Jump Boost.  They don't heal much because of that, otherwise it'd be too unbalanced.