The value of your loan must never exceed 66% of your collateral. Put another way, the value of your ETH collateral must stay above 1.5x the value of what you borrow.
In Argent, the maximum loan you can take is 60% of your collateral. This is to allow you an initial buffer. But once the loan is open the ratio can change, so you should keep an eye on it.
What might cause the Loan to Collateral ratio to change?
The value of your collateral may fall due to Ethereum market fluctuations.
Also, your interest accrues over time. When you decide to repay the loan, the interest due will be deducted from your collateral. If you borrow the maximum 66%, the additional interest will quickly tip it over the limit.
If this happens, your collateral will be used to repay some or all of the loan (liquidated) with a 13% penalty.
We recommend you keep your loan to collateral ratio low, and monitor it regularly.
Maker have listed several risks to their platform, along with their steps to mitigate them. These include plans to defend against hacks to their smart contract infrastructure, limiting the debt ceiling, competition with centralised counterparts, keeping the Dai supply steady, and incentivizing the Maker Foundation.