The issue with FRMS is that it is whatever you want it to be. If the company and pilots are understanding and flexiable it can work well. However you can get some pretty ridiculous results out of them which the response is 'well it fits into the FRMS OK'.
It is then up to a commitee to agree on whether something is actually dangerous or not. The strength or weakness of your fatigue committee is the achilles heal of the system.
The other gaping hole is that FRMS doesn't consider cockpit noise and vibration, which is pretty damn high in some aircraft.