One of the draw backs of the profitablity index as a criteria for judging whether to accept a capital investment project is that:

It might lead to over-optimistic projections

Explanation:

In simple words, the problem with using profitability index as the index criteria lies with the procedure of estimating it. In order to consider the business situation, the organisational finance group requires to settle with the corporation supervisors.

Leadership may be too enthusiastic about their assignment, so forecasts for cash flow may be too substantial. Consequently, in predicting the profitability index, there may be an uptrend prejudice.