When you unfurl you maintain a bit of tension on the furling line so it doesn't bunch up inside the mast and lays properly. Similarly, when furling a small amount of tension on the clew line gives you a nice tight furl. The tension required can generally be achieved by a firm grip on the line and letting it slide through your hand, or by a single turn on a winch and only enough hand pressure to keep the line on the winch (note doesn't work with Andersen winches since they grip too well even with only a single turn).

I have sailed quite a lot on boats with in-mast and never had a problem except one one boat that was chartered and the last charterers had rolled the sail in very loosley. Once we got the sail out and rolled it up properly everything was fine. Once you learn how your boat's system works it will be easy. Remember, there are people that can screw up a roller furling jib.

The negative of in-mast is sail shape. The battenless sails are generally cut pretty flat. The drive you get from an in-mast main can be quite disappointing particularly if you are used to a full batten sail. The sails perform poorly particularly when they are a bit old and stretched out, just like any sail, but since you are starting from a comparatively poor performance point to begin with, an older sail may seem almost useless.