Mixers are a terrible idea in general. Never give someone you don’t trust unfettered access to your coins. I’d give the advice I would give with gambling. Don’t risk anything you are not fully prepared to lose.
Anybody should always be cautious when using a new platform, no matter it's gambling, trading, mixing related.
One should be careful and try to use, where possible, reputed platforms and never with life changing funds at once.
That's OK. But from normal cautiousness to labeling
all mixers "a terrible idea" is a big step in the wrong direction.
I'd say that your statement is pretty much similar with telling "never use any exchange because not your keys, not your coins".
PS. Some may say that my statements here are biased. Well, I would say the same even if I would not wear this signature, but one can believe what he wants; this is the beauty of the freedom of speech.