Many costs are not fully variable, instead having a fixed cost component that must be derived and then included in the flex budget formula.
A great deal of time can be spent developing step costs, which is more time than the typical accounting staff has available, especially when in the midst of creating the more traditional static budget. Consequently, the flex budget tends to include only a small number of step costs, as well as variable costs whose fixed cost components are not fully recognized.
The flexible budget model usually only works within a relatively limited revenue range; the budget analyst is unlikely to spend the time developing a more wide-ranging model if it is considered unlikely that outlier revenue amounts will be encountered.