Multiple engines complicate design, because not only must the airplane be able to maintain flight on one engine, but also the pilot must be able to control it. Lateral/directional stability issues are key;
Some builders of Experimental/Amateur-Built aircraft are builders in name only, deferring primary construction of the aircraft to the pros. So concluded the FAA, which is determined to clamp down on those who would flout the 51% rules. What does it mean to regular homebuilders? Those who are building now will be unaffected, but for future builders, the FAAs guidance revisions could be significant.