Not great, but worse case is they'll just reject you from participating. You can use a different coordinator if I'm not mistaken although in practice this seems difficult to find...
Discussion
I would argue the worst case is that any mixer doesn’t actually provide privacy but people think it does, e.g. law enforcement is Sybil attacking plus most people eventually dox their own coins.
Yeah… this type of privacy does not scale.
I don't know about not providing any privacy, but you're right privacy could definitely be reduced from what we expect it to be...
You could always use Monero to "mix". While it's sender privacy could potentially be just as reduced by Sybil attacks, amount and receiver privacy are not affected since they rely on encryption via CT and Stealth Addresses. No centralized coordinators or extra fees either.