Not all plants are created equal at all. Some, may be easy to grow and require a minimal amount of upkeep to maintain its growth, while others need a lot of help in order to grow fruitfully. This is important to know when considering which plants to grow in your own personal garden. The tomato and the onion are a classic example of this.
One of the easiest plants to grow under the sun is the tomato, a plant that needs a very little attention to survive. It only needs about two inches of rain a week to survive, and that is a very slim amount. They also grow very well in some hardy conditions, they are accustomed to almost any area that receives rain. They need around 7 hours of sunlight a day as well in order for them to thrive and bear the best possible outcome. This differs greatly from the onion. The onion is rather challenging to grow for a new farmer.
The tomatoes my old school grew 
The onions I grew
Onions must be placed in a little over an inch of dirt, and they need around 5 inches of space compared to the few inches tomatoes need. It survives in zone 3-9, while a tomato can survive zone 5-9. It needs a long time to grow, and only needs an inch of rain to grow. They are so difficult because they do not have the best defenses against pesticides and insects.