Having a mixer be detected could potentially mean bad opsec from the mixer's side. It's true that oftentimes mixers are used by people with notorious purposes, but the issue here is that the mixer can't simply disregard coins it received from people that end up making illegal transactions that could even be traced later.

However, the mixer's mistake lies in the fact that they didn't bother to actually "mix" the coins enough so as they can't be detected from whatever basic heuristic detection an automated exchange could be using. So by warnings like these I would indeed try to stay away from a service that doesn't fulfill it's basic function. A mixers job isn't just giving you coins that aren't yours. It's to give you coins whose origin isn't so easily detected... If they fail at that it's like a plane that can't take off. Seems nice but not very useful.

It seems as though OP learned the hard way that some mixer services simply don't care to output coins that are even more tainted and more easily traceable than what you might have put in... But that's not to say that the perfect mixer exists. Bitcoin tracing methods have been getting better. Oddly enough in the end using unorthodox methods like withdrawing from an exchange makes for better and also much cheaper mixing..