I worked in consulting for over a decade, and... yeah. When people migrate from one system to another, they try to make the new system work exactly the way the original system does, especially if the original system is homegrown. That lack of flexibility tends to be responsible for more than half the cost (and time span) of the migration.
There's also the related failure mode of deciding you're going to fix all your other system problems as part of the migration, bloating the scope and creating too much complexity. Just do the necessary stuff, migrate, and then go back and fix the nice to haves.