A project often depends on software produced by organizations that it does not control. There are great risks associated with third party software that must be recognized by everyone involved.
Never, ever, rest any hopes on vapour. Vapour is any alleged software that has been promised but is not yet available. This is the surest way to go out of business. It is unwise to be merely sceptical of a software company's promise to release a certain product with a certain feature at a certain date; it is far wiser to ignore it completely and forget you ever heard it. Never let it be written down in any documents used by your company.
If third-party software is not vapour, it is still risky, but at least it is a risk that can be tackled. If you are considering using third-party software, you should devote energy early on to evaluating it. People might not like to hear that it will take two weeks or two months to evaluate each of three products for suitability, but it has to be done as early as possible. The cost of integrating cannot be accurately estimated without a proper evaluation.
Understanding the suitability of existing third party software for a particular purpose is very tribal knowledge. It is very subjective and generally resides in experts. You can save a lot of time if you can find those experts. Often times a project will depend on a third-party software system so completely that if the integration fails the project will fail. Express risks like that clearly in writing in the schedule. Try to have a contingency plan, such as another system that can be used or the ability to write the functionality yourself if the risk can't be removed early. Never let a schedule depend on vapour.