If I accept a contract-to-direct assignment, is direct employment guaranteed?

No. Although the client has the intention of converting you to a direct employee if the contract period goes well, this cannot be guaranteed. There are several reasons why a contract-to-direct assignment could be terminated without an offer of direct employment. These include the loss of a key project, the loss of funding, an unexpected downturn in the economy, etc. That stated, in our experience the vast majority of contract-to-direct assignments do lead to direct positions.