Internal RC clocks typically don't have the accuracy & stability of an external crystal or other clock source.
So check the datasheet carefully; if the specs meet your requirements, then the internal oscillators will be fine - if they don't, you'll need something else.
This is particularly important for things like UART - especially at higher baud rates.
A complex system that works is invariably found to have evolved from a simple system that worked.
A complex system designed from scratch never works and cannot be patched up to make it work.
