Mixers are not illegal by law but if a mixer is used by sanctioned entities like Lazarus group from North Korea or IRGC from Iran, that mixer will turn to illegal and face with seizure from governments and some agencies.
There is no definite mixer law, FinCen has stated that the mixer is not illegal as long as it prioritizes privacy, this area is also still unclear whether we use this mixer is still safe if in the end the coin ends up on the exchange? Often exchanges hold coins that come from mixers.
Hackers are now laundering more money on TornadoCash, which means that mixer is not as full of crime, right? Still, for me using a mixer is still illegal, not a matter of privacy.
About one year ago, eXch.cx operated well before their service used by Lazarus group and it was seized by government. With mixers, the verge between legal and illegal operation is very small, and it's hilarious if mixer KYC their users.
Mixer implements KYC or AML checks then this is not much different from other instant exchanges, I don't think mixer will do that.