Well you are laundering money (which is likely illegal depending on your reason for doing so), so it'd be unreasonable not to expect there to be some risk. You're just trading off risk from government for risk from mining pool. But if you do the laundering over a series of transactions spread out in time, you are limiting your potential losses. Say you do it in fifty blocks over a week -- if the miner betrays you your maximum loss is 2%. If you don't tell the miner how much you're laundering through them (they'll just know when you are finished), and you give them an acceptable cut (say, 3%), it would be in their vested interest not to betray you because the expected value of more laundering from you is greater than what they could make by eating any one given attempt.

Also, if they are doing laundering as a service, then trust in them is very important, and screwing over one customer at the cost of potentially losing all future customers is not worth it. For instance, let's say I'm selling $5K of Bitcoin. I'm not worried too much that any given exchange is going to screw me over and eat it, because the big exchanges are doing many millions of dollars in business a day, and stand to lose a lot more from a hit to their reputation from stealing from me than they do to gain from eating my money. The only worry is if an entire exchange goes down (a la MtGox), but you can minimize that risk by not keeping money in an exchange.