No Easy Job
The Harrier is attempting to do something that fixed wing airplanes were never meant to do. By the very nature of what this airplane does, it is going to be a dangerous airplane to fly. The Harrier was also the first generation of fighters to ever do this, and a lot of lessons have been learned that will benefit the F-35.
With the configuration that the Harrier (and Boeing's failed X-32 prototype) uses, there is a very real risk that hot exhaust air from the lift nozzles can be sucked back into the inlet. This can stall the engine, sending the airplane from "hover" to "drop" mode in an instant. The F-35 avoids this by using a separate lift fan to support the forward half of the airplane's weight. There's no hot engine exhaust to ingest.
The Rolls Royce Pegasus engine that powers the Harrier has also had more than its fair share of growing pains - including recurring problems with titanium fires. Once you have metal catching fire on board, its ejection seat time. There's nothing that's going to put it out.
Military hardware by its nature is going to see hazardous duty, even in peacetime training. The Harrier was just pushing the technology envelope further than it could reliably be stretched at the time.