Until you've got that legacy system that the project has to integrate with. At that point, the legacy system is dealing with twenty different new large projects trying to integrate with it, instead of one.

MVP doesn't work well with deep integration. You can break this down to a form that takes one input, and returns one result on a following page. From a UI perspective, this seems like one Agile story. But that one round-trip can spawn so many integration steps. I just got finished with a health care IT project like that. One round-trip step involved integration with a single-sign-on service (which needed to be reconfigured), a rickety SOAP service provider (which had limits in how many test boxes they could set up and was controlled by a different bureaucracy and needed approval processes to turn on each required API method), a separate box returning chunks of patient data wrapped in html (don't ask, this was again out of our division's control), and our own backend system through REST so our resultant data would not be stored on the same server as our webserver (cluster). If some of these backend servers were told "okay, you now have twenty implementations to deal with instead of one", it would have drastically reduced the probabilities of completion.