Having operated five early flights on a UK CAP 371 roster I am happy to testify that by day five I was really tired and often didn't make the end of the pattern. It is seemingly impossible to keep shifting your sleep pattern in an anti-clockwise direction.
It intrigues me that Ryanair pilots don't seem to complain of problems with fatigue (or over tiredness) with the pattern. However, from what has been discussed here, I wonder if that is because the pattern is (apparently) very predictable. I.E. the pattern of five earlies on a CAP 371 roster may appear at random and be mixed in with other awkward shifts. The Ryanair deal allows for eleven days recovering your sleep pattern. This would seem ample.
There seems to be a slight advantage in coping with a tiring pattern when it is what you expect. It is obviously less stressful on your private life if you can make certain plans for normal social arrangements.
Nassauman makes the amusing point that 'the FR system works in practice but doesn't work in theory '.
CAP 371 is a tortuous system to work under with some airlines; we all know that it is used as a work target and that creates the problems it is written to circumvent when sometimes a locally agrees deal suits both sides.
The Ryanair deal sounds pretty good for both sides by all account. Presumably it functions in this niche because there are neither night stops or night flights. Include these variations to your body rhythm and 900 hours a year really would cause fatigue.