The phrase often implies that the reality is much more extreme than described.

Understatement

Use it to hint at strong feelings or situations without being too direct.

Subtle Intensity

It's a polite way to express criticism or negative impression.

Indicates that you're consciously choosing to describe something with restraint.

Suggests there could be a stronger way to express thoughts.

Fits well in informal conversation, indicating a hint of sarcasm or irony.