The "Rutan problem" came about from their use of enhanced laminar flow aerofoil sections. The rain upset the boundary layer causing early transition at a lower Reynolds number. This caused a large reduction in lift and increase in drag, some Ezes could only just maintain level flight in moderate rain. Roncz subsequently worked on a much more tolerant section which largely eliminated that problem.
One of the main targets for enhanced laminar flow on airliners has been on engine nacelles, so I suspect there may be a rain factor on their performance.