That statement is particularly true regarding the spinnaker. The problem is not to hoist the sail but to get it back on the boat. That's when the spinnaker sleeve makes the average sailboat owner a great sailor. Without any help, he makes that very powerful and expensive sail disappear with a minimum of fuss and trauma. And the sail is ready to be hoisted again immediately, no more packing!
While it would seem that a spinnaker sleeve is a simple sailing tool, and that any one with a sewing machine could design and fabricate one, when dealing with large sails, the problems increase rapidly in complexity and cost. It is very plausible to get hurt or to inflict great injury to others, and to one's wallet, if the system used is inadequate. But there is no need to rub it in, especially at sea: it has to work and be reliable.
The main problem to solve was the mixing of the control lines and the sail.
A separate channel (which contains the control line) is sewn alongside the main sleeve which contains the spinnaker. The spinnaker and the control line are completely separated . The separate channel (side sleeve) is made of a different colored cloth than the main sleeve which shows the eventual twists of the sleeve prior of hoisting it. There will often be some twists in the sleeved spinnaker as it comes out of the bag, especially after long periods of storing and moving it around the sail locker. Needless to say, those twists are to be removed before hoisting the sleeve.