They are suitable for use only when uncertainties involved in contract performance, such as research and development work, do not permit costs to be estimated with sufficient accuracy.
They are only useful when uncertainties are involved with contract performance. 
mnli hypothesis: They are only useful when uncertainties are involved with contract performance.  premise: They are suitable for use only when uncertainties involved in contract performance, such as research and development work, do not permit costs to be estimated with sufficient accuracy.