The answer to this question basically depends on the attitudes of the people involved: you (as a project leader), your organization/leadership, the body requesting the evaluation and the external expert who helps you do it.
Your attitude: If your organization, project, unit evaluates just because somebody else (donors, board, supervisors) said so, you see evaluation as a necessary evil to get more funding in the future, you collect data to fill in required forms and you make sure to be seen in the best possible light, if the people who require the evaluation punish you for admiting mistakes and obsess about their forms being filled in properly… well, evaluation will not only be a pain and an annoying and scary exercise where people bend the truth as far as they can get away with, it will also be a cost. Or shall I go so far as to say: a waste of money? Well, you might get some benefit out of it, for example the future funding you were after, but you will loose out on the opportunity of learning and improving based on experience and analysis.
If, on the other hand, you can develop the common understanding that the struggles your project went through are learning opportunities, that everyone involved has something to offer to understand what happened in the past and how to do things even better in the future, if the people who request the evaluation require measureable outcomes, but also know that obstacles overcome make you stronger and that you sometimes need to change your direction in the middle of a project (because of changes in the world or learning processes within the project), then your evaluation will be an investment, it will be an amazing and empowering learning experience for everyone involved and catapult your work to the next level in the future.
If your organization sees evaluation as a cost, you might not want to call me just yet, because my tools and approaches won’t work for you. If, however, you want to turn your evaluation into an investment for your future and a transformative learning experience for everyone involved (while still collecting data about your performance), we should talk.