Also, although underside might be a word, in all my life, I have never heard it used in conversation or read it (until now). A more common word is underbelly, which means "vulnerable to attack" or the "criminal side of society."

"Up" and "down" are directions of motion, but "under" is less so. So "down side" refers to an aspect that will carry you down. "Underside" carries you nowhere. Instead, it makes you think of the soft under parts of an animal, like the tummy of a dog that is sensitive and easily hurt.

Here's a more common use of the word underside: "After hitting the speed bump the underside of the car was damaged". In this context it would describe the bottom of the car. I cannot think of an example to replace downside with underside.

Underside is rarely used although grammatically correct. Downside or drawback are better choices, and both are used interchangeably.

Advantage/disadvantage Upside/downside Benefit/drawback Pros/cons These generally go together when you are discussing positive and negative aspects of something. Underside is more of a general negative feeling about something than a specific element.