I'm a sailplane, powerplane and a paraglider pilot and experimental airplane owner, with two decades of soaring flight behind. I have a micro-scale meteorology and atmosphere modeling as a hobby. I work as an engineer and computer vision researcher, on autonomous vehicle project.
My expert opinion - for limited delivery applications sailplanes could work. You need to do it right - sample and model atmosphere from the vehicle and ground, use gravity to accelerate and decelerate to flying speed, complete automation to remove the pilot. It's not easy. But it is not impossible.
(Incidentally, if someone is willing to drop me ~500k in an attempt to build a prototype - I might consider investing the time required.)