It's been several weeks now - they replied once to ask for a letter of guarantee, and since then I haven't heard anything and my bitcoin is lost - it's a small amount so it's okay but at this point I'm considering them to be a scam in that they send some bitcoin over but not all.

I sent over a small amount as a test on [banned mixer] - I did not receive one of the transactions. I e-mailed support and they asked for the letter of guarantee. Sent it to them but no further response after that

So I lost a small amount and the mixer does not seem reliable.