Speaking from my experience of working on Alfresco projects since 2010, I have never used any "standard" / reusable migration tool in any of my customer projects. Typically, the requirements were way too trivial / simple to warrant the expenditure of several thousand Euro to buy / rent a migration tool, and then spend some more money to configure / cusomize the tool for any freaky / special requirements, when the same purchase-only price tag could be used to build a quick-and-dirty export/import script ourselves.
Migration tools become interesting when you exceed hundred thousand(s) of elements, the migration has to be performed continuously (e.g. gradual transition with overlap where two systems are active at the same time), and/or there is a substantial need for systematic processing (transformation / cleanup / metadata inference). Outside of a big financial services company or some government institution, it is generally rare to encounter a big migration project.