Carmen has to work within the parameters set by the airline; obviously it's main priority is going to be work coverage - airlines are not flying clubs. If your airline is undermanned and operates flights at unsocial hours (name one that's not like this) then Carmen is going to produce some unpopular trips and the pilots are going to be working hard. How much the pilots fly is not the responsibility of a computer programme.
I have used Carmen from a junior and senior position within my fleet (BA LGW)for several years and am generally fairly satisfied. I have not used bidline and I'm prepared to accept the consensus of those who have used both that bidline is better than Carmen. Some people bid for days off and some for style of trip (late/early, daytrip/nightstop or particular destinations), but the key from the user's point of view is to keep bids simple. Trying to construct your own roster by 'clever', complicated bids does not work. I generally get what I want and there are some people with whom I very rarely fly, because they bid for the opposite from me (nothing personal, you understand!). So Carmen must be pleasing some people.
BA does not use Carmen for allocating leave. The company at LGW also operates a system of trump days: four separate, 2-day blocks per pilot per year allocated in a book on a first-come, first-served basis. So you can accept a wedding invitation in 6 months' time in complete confidence. This manual system outside Carmen mitigates the uncertainty of bidding for days off within Carmen and is a very popular adjunct.