Have a couple of unique migrations that don't come along to often. They are Unidata and RDB migrations. The top three things that make these migration more challenging are:
1. No automated data migration tools - Because these migration don't happen that often, there are no tools in the market place to automated the data migration.
2. Application is tied to database - The application needs to be re-architected/re-engineered. Unidata Basic and COBOL for RDB. TSRI can migrate Basic to Java and PL/SQL. Transoft can migrate DEC COBOL to Java.
3. New client hardware potentially involved - Many Unidata and RDB based systems use 'green screens' as the front end. These are character based screens that will run on very old dumb terminals such as: Wyse and DEC 5250 terminals. The user interface can be replicated in a web browser but many times these old terminals do not support web browsers.