System implementation using one or more databases. Pros and cons.

What are the advantages and disadvantages of using one or many databases? 

Single environment:
+ Pool of licenses.
+ Only one environment to manage.
+ Only one place to login, common user experience.
+ Lower costs of adding common modifications.
+ Financial data consolidation.

Multiple environments:
+ Less complex testing.
+ Less risky in terms of server failure.
+ Less complex decision making.