Usualy the ones i have seen available for free have the core functionality but no perhiperals,timers,uarts, interrupts, whatever so its upto you to provide those and plug them in, or in the case of the one core ive seen extensivly rewrite it so that interrupts do actualy work.
The other problems are things like they are not optimised for speed and are written in a way which is not conducive to them being easily edited.
Missing documentaion,no testbenches and lack of support all add up to meaning that taking any of them on involves a lot of work to get them working.Converting them to work as a mutistage pipelined design is not something to be taken lightly.
Having said all that they can be a good starting point for a minimalist core with the potential to be very fast and support only those features that you really need. They are not for beginners and dont be surprised if they dont actualy do what they say on the tin because quite often they are abandonware which somebody has simply given up on.