Use of any one mixer comes down to trust. Trust that the mixing is sound, that the mixer is not compromised, and that it's not a honeypot to begin with.
Using two chained mixers mitigates most of this risk.
Let's say there are six mixers.
And one of the mixers is a parrot.
Now the odds of hitting one that is a parrot goes up the more mixers you use.
A better way to try to convince people to use a mixer, is to show with analysis that multiple mixings or mixers show greater [whatever desired reason for a mixing to start with].
If one of the six mixers is a parrot, it doesn't matter. It ony knows that it's mixing coins for one part. Could be mixing a mixer's output and sending to a mixer's input, know your source address, or you destination address. All mixers would have to be linked parrots for there to be an issue.
Mixer analysis has already been done on most of the individual mixers. If either mixer gives a good mix, there should be no taint. The mixer knows your source and destination, if you only use one and it's compromised, there's an established link between your coins that the parrot mixer knows about.
If you were randomly using different mixers in parallel, it would be much worse than a single mixer. Series is where there is the advantage.