I've read that the extra lift generated by the nacelles doesn't actually result in aerodynamic instability -- the plane's not going to stall itself -- but results in reduced backpressure on the yoke in a way that fails airworthiness certifications for required amounts of pressure. (With the idea being that the pilots may induce a stall due to not needing to provide as much force on the yoke as they would expect to have to in order to do that.)