Canards
The fore plane produces downwash over the center of the aft wing and a vortex in the middle of each aft wing panel. This disturbed flow over the aft wing is worst at slow speeds below the best L/D, where sailplanes spend their time in thermals. At crusing speed and above, this is less of a problem and can be partly corrected by washout from the foreplane vortex to the tip of the aft wing. Hence the Voyager's ability to circle the earth without refueling. The amount of washout compensation varies with airspeed ( angle of downwash). This makes it extremely difficult to achieve efficient aft wing lift distribution (low induced drag) over a wide speed range.
Canards must have both the high speed and low speed lift characteristics of the foreplane and aft plane coordinated to avoid potentially disasterous handling problems. The fore wing must stall before the aft wing for favorable stall handling. The aft wing must go through zero lift before the fore wing to avoid violent, uncontrolled pitch down with negative G maneuvers or certain gust conditions.
A genius like Rutan can accept the risks and overcome them, unlike the design team of a large aerospace corperation.