You use to put it mildly to indicate that you are describing something in language that is much less strong, direct, or critical than what you really think. 说得委婉些
adverb [程度副词]温和地,和善地;适度地
But not all the money, to put it mildly, has been used wisely. 但是说得委婉些,并不是所有的钱都用得很明智。