But the burden of proof on what? That my opinion is that there are centralized mixers that accept the trade-offs of being centralized/be a trusted-third-party to do what their service is supposed to do?
Every custodial mixer is closed source and therefore we really don’t know if they are doing what they are claiming to do. Many of them don’t make a serious effort to provide users with privacy and are just clumsily moving coins around between wallets without any concern that they might be leaking information that may potentially deanonymize their customers. Mixers are really just a cash grab foremost, rather than a privacy service.