No experience of a behind mast furling main, but I did have an in mast furling main when I first bought the boat. Some may be relevant
1- The sail shape was awful and there wasn't much you could do about it - no roach, no battens. I'm a bit of a tweaker when it comes to sail shape and so it drove me insane not getting to anywhere near acceptable. I've seen some attempt vertical battens, but without tremendous gain in my opinion.
2. Clunk, clunk, clunk every night at anchor
- I have a pullman berth with my head
next to the mast. every time the mast went over the vertical, in the smallest of ripples, the rolled sail clunked against the inside of the mast. A sound at about the same annoyance level as a slapping halyard
3 - I didn't actually find it all that much quicker / easier to reef than normal slab reefing - it was certainly more temperamental. It saves you putting on a sail cover
I suppose - but I don't find that a huge chore
4. I did actually get in the position where it wouldn't roll in when I needed it to. Then the worm that the furling line winds around sheared off and then the whole sail came out. Which was a good thing because I could finally get rid of it.
So now I have a conventional, slab reefing main and I am a happy man....until I sail downwind in some decent wind
, then the furling main comes back to haunt me with a eerie howling from the slot in the mast that used to be its lair.