This is a delicate problem.
You are the bride's best friend and you are not comfortable with her choise of husband. The wedding is next week and the the question is if to say anything or not.
It's similar to the situation when you work in a larger project and you know by heart it is not going to fly. The problem with the program is twofold. It's both the timing of the message and who says it.
To make changes in an organisation and IT-systems takes a lot of effort and it's always easier to say no. It is very natural to say that this new thing will not work. If you are part of the existing organisation, then you may be accused of being a pessimist that don't like changes. It's not easier if you are a consultant who have worked with the old systems. Then it may be in your self-intrest to preserve the legacy and that could be used against you.
So if you are brought in as an expert from the outside, they may be listening you and what you have to say.
But, if you are in charge of a large program, and have the trust from top management. Why bringing in some from the outside? You are given the task to manage the program and you know how to succeed as you done large programs several times before or this is your opportunity to do a career. You are optimistiic by nature and success is something you and the steering group expects.
If you are the bride, will you listen to advice from someone else when you are in love and plan for a beautiful and fantastic wedding? Probably not.
If you as a project member see the clear signs of failure in a project, the first thing to find out is if the person in charge is receptive to bad news. If not, you will become yet another pessimist or partyspoiler unless you wait to the right moment. If it's possible for her to listen to arguments, you have to be well prepared with both a really good story and facts that can back it up. It is also a good advise to have some idea how to solve the issues. It's is a lot about risks, probability and the need for change. Sometimes, you don't have any options but run the project.
if you have responsibility for a part of the program that you don't belive in, then you have a dilemma. Saying no may harm your career and say yes may put you in the same position after a failed project.
My experience, as a professional, is that is always better in that situation to flag for the issues and be very clear about what you can take responsibility for or not. Then it's up to the persons above you in the organisation to decide the way forward. Eventuellt, they need to find another person who would like gamble on the outcome.
The danger with not handling identified risks is that the motivation in the project decreses and that the productivity falls below expectations, thus making it even harder to succed as promised. Therefore, the program management must be aware of the challenges with change.
"if anyone can show just cause why this couple cannot lawfully be joined together in matrimony, let them speak now or forever hold their peace"