Ethereum's consensus model makes the same security guarantees for contract operation that it does for the built-in currency; given the value of Ethereum-based tokens there's a very large bounty for anyone able to poke a hole in that.

Of course this is only a guarantee that contracts will run as written, not that they're written correctly. There's a lot of research on applying high-assurance techniques to contract development but it'll be a while before that stuff is ready for production.