If you had a go at switching to CakePHP, you are probably aware of the conventions to follow to get the framework to provide its ORM features. Sure, FuelPHP has conventions too but they are simpler, and the ability to configure what conventions you would like to use or even override those conventions are well-documented.
There still is a bit of rigidity in naming controllers but you can name your database tables and columns as you please and then define how FuelPHP should treat those columns. Having said that, I should also mention that FuelPHP does not make the configuration of the ORM as complex as Hibernate does.
There is also the ability to have FuelPHP auto-discover columns and leave most of the heavy lifting to the framework while you are prototyping.