In 2008, the FAA issued SFAR in addition to the type rating. Specifically, special waivers were required after satisfying training two potentially fatal flaws in the a/c's performance. Engine out, flaps are required to stay out, with a lowering of the nose. Additionally, an Auto-pilot was required for single pilot ops. Landings requirement for currency must be performed in the MUMU, no other type.
I couldn't find NTSB's recs. but would assume they had identified the weak spots, and suggested the a/c not fly w/o upgrades to training, and equipment. NTSB is a board, and submits advisories only. FAA for rules or ticket lifting.
bear