Literally any mixer could freeze any order for whatever reason known to man. And it's not like this hasn't happened before.
From admins going AWOL to simply not caring to maintain their systems, all the way to outright scams. Mixers are also a 100% centralized and trust based system. There's not any documentation proving that they're better at what they claim to do than a simple exchange.
I totally agree with you, mixers are a very risky service as a lot of people have pointed out on this forum, you lose custody of your funds during the mixing process and you have to trust the mixer to pay to your receiving address, it is similar to exchanges on the basis of trust, you could deposit your funds in any of these services and lose it, because you lost custody of it. People who use mixers should be aware of this, and if you want a self custodial privacy protection service, use CoinJoin.