to say the least

used to indicate that the statement being made is a significant understatement or that there is much more that could be said about the topic

The traffic during rush hour was a nightmare, to say the least.
His behavior at the meeting was inappropriate, to say the least—he was rude and disrespectful to everyone present.
The movie was disappointing, to say the least.
His performance in the game was lackluster, to say the least—he made several critical errors that cost us the match.
The situation is concerning, to say the least.