I have only seen posts from people who have used mixers or people who received funds that were associated with a mixer, such as those in the Sinbad campaign on Bitcointalk. Anything which they think is suspicious might cause your funds to be withheld until you can prove their origin. Their methods of detecting what they consider tainted funds aren’t entirely reliable so people often try to deposit funds from hacks and phishing scams directly to exchanges, hoping they can go unnoticed long enough to cash out in fiat.