Ever try adding a small grating in front of the reel mowers to catch debris and push it to the side (think cattle catcher on a steam locomotive)? Or maybe an auger, like on the front of a combine, to direct debris to the sides? Or invert the auger to direct debris to a small conveyor belt in the center to collect debris into a container (pine cones friggin' hurt when they make it out from under a rotary lawnmower)? Or a 'jamming' gripper on a small-ish arm to grab debris sighted by a camera observing its path (collect or throw away)?
Can you tell I've wanted to build a robotic lawnmower? I had once considered using a reciprocating-type clipper akin to a hedge trimmer, but that seems to have the worst parts of both a reel and rotary mower (still needs some sort of protection like a reel mower and a power source like a rotary mower).