Multisource universe in BOE 4.0

Hello All - Can you please help me answer the following based on your knowledge and experience with BOE 4.0 Universe.

Questions

1. What is the complexity of having a universe built from Multiple sources (ex - SQL server, Oracle etc..)
2. Keeping SAP best practices in mind, from the design prospective when dealing with multisource database reporting, does it make sense to keep different universes for individual database and merge them at reporting, or leverage the new functionality in BOE 4.0 and bring in all datasources into one universe.
3. If I have a universe object created on Oracle data base, and in the future i repoint this object to a different database, does that change the CUID.