I have the sense that migrating to any hermetic, cacheable build system will be more effort than migrating from one to another: once a project is “safe” to have everything run in sandboxes, the mechanisms for setting up those sandboxes is less important. For example, once one system has accurate dependency information, that can be used as a base for setting up the second system (and presumably this info can be exported for machine-assisted migration).

Of course, the details of each system differ enough that I imagine it’s not trivial.