According to the documentation, RSEC_MIGRATION is approximately 80 % successful. Specifically, what does not work (other than customer exit variables that it mentions)?
The full list of what does not work can not currently be provided since the definition of authorization objects are quite individual. One example is not checking key figures for some InfoProviders. In the 3.x concept, this can be achieved by not including key figures in the authorization object. Migration takes existing authorization and migrates from authorization objects to analysis authorizations. Since there is no authorization for key figures, but key figures can be authorization-relevant, InfoProvider is checked. The check would fail which was not the case before.