Vertical flight worked. Forward flight worked. The rotation between them never did.
Search and rescue wants an aircraft that launches without a runway and still cruises efficiently. A tailsitter promises both, and concentrates almost all of its difficulty into the rotation between them.
Carried the mechanical design and its redesign, built and flew 4 prototypes in composite-reinforced foam, and produced 5 iterations of the power distribution system - component selection, soldering and validation. Bridged a Pixhawk flight controller to the QGroundControl ground station for autonomous flight.
Vertical flight worked and forward flight worked, each demonstrated properly on its own. The transition between them never did.
The aircraft repeatedly failed to build enough airspeed during the rotation, and prototypes were crashed that way. The diagnosis named the fixes - rotate more slowly, add an airspeed sensor so the manoeuvre is flown on measured speed rather than open loop, reduce weight, shift the centre of gravity aft - and none of them was carried out before the project ended.