Downgrading

Downgrading is usually not recommended.

Nintendo 3DS
In the past, downgrading the system software was necessary for homebrew. The main reason most people did it was because arm9loaderhax could only be installed from 2.1.0-4. That hasn't been true since at least 2017 though with the release of boot9strap.

Side effects
If you do attempt to downgrade anyway, you can run into issues:
 * Unable to boot if performed incorrectly.
 * Unable to access online services.
 * Save data compatibility issues.
 * Games not launching.

Nintendo Switch
Downgrading your firmware on the Switch is not recommended. This will generally lead to a lot of issues and won't solve anything.

Side effects

 * Unable to boot if performed incorrectly.
 * Unable to boot due to a mismatched efuse count.
 * Inability to use your gamecards. The card slot has its own firmware that is occasionally updated, rendering it incompatible with older versions.
 * Save data compatibility issues.
 * Games not launching.

Wii U
(Wait, can you even downgrade a Wii U?)