The prior owner (an engineer with metals and fabrication expertise) designed and built a behind the mast roller/furler for our boat (a Hunter 34).
He used a Harken unit with custom brackets. The sail was purpose made by a local vendor to optimize sail shape and still be easy to roll up.
This thing has been on the boat for over a decade and works perfectly! Not one failure that wasn't operator error.
Yes you lose some sail shape but not that much on this boat. We still move pretty well.
The weight aloft is there but does a couple of pounds really make much difference? All we have is a windlass and antenna at the top of the mast.
It is fantastic at the end of the sailing day when we grab the furling line and put the sail away in less than 15 seconds. We find that we use the sail more often than our neighbors who have to go thru the gyrations required to unwrap and put away their regular sails.
I couldn't be happier with our behind the mast furling system.
FWIW: I have all the engineering drawings in case we need to have replacement parts fabricated.
I have to hope that Pilot meant to write windvane
The behind the mast furler that Pilot describes does have some advantages. Cost being a primary one. The cost of the custom fabricated pieces, the new stay, some blocks and the furler has, in my experience, been a lot less than some of the other options.
An in-boom furler would probably be at least twice as expensive.
This is another one of those issues that depend a lot on many different factors. The age and psychical condition of the owner being a big one. The type of boat being another.
I've installed these systems a number of times for people who were finding it increasingly difficult to handle a convention mainsail. None of these people were racers.
If it can keep someone sailing for awhile longer who might otherwise have had to give it up, then it's a good thing in my book.
There is no reason that I can think of other than operator error that should make a BTM furler any more prone to failure than one on the headstay.
Granted, it would be a lot more difficult to wrap the sail up by hand if you had to but again.
If it's installed correctly, maintained correctly and used correctly, roller furling, no matter where it's installed, is pretty darned reliable.