What happens if I open a "Session" and transfer BTC and then the site gets attacked and is not available for a couple of days?
BTC gone?
Could potentially be the case if you didn't have your session token written down somewhere. There is really no way for you to prove that you were the one that actually deposited the bitcoins especially if you're operating from tor.
But perhaps Chipmixer will on a case-to-case basis refund peolpe that experience this problem.
It's not guaranteed though - which is why it's always the safest thing to do if you save your session token.