The biggest risk is not estimating the work you will do, but documenting the dependencies on things the client must provide in a timely manner when you need them, and what the outcome will be if they are not delivered (eg, do you expect to be paid for doing nothing if blocked?).
These dependencies are often very difficult to articulate to a level of detail that gives you the protection you'll need should it come to that, and it is even more difficult to ensure you capture all potential dependencies.
I'd go for the recommended approach of the first phase being a scoping and estimating phase, with defined outputs such a phased stage plan with defined deliverables in each stage. You can then work to fix price each stage.
These dependencies are often very difficult to articulate to a level of detail that gives you the protection you'll need should it come to that, and it is even more difficult to ensure you capture all potential dependencies.
I'd go for the recommended approach of the first phase being a scoping and estimating phase, with defined outputs such a phased stage plan with defined deliverables in each stage. You can then work to fix price each stage.
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