It has been done, but I've read other posts on this board mentioning that it isn't always done correctly.
If you get a real 2" receiver hitch (vs. a 1-1/4" with an adapter) and have the flexibility to mount the thing very, very close to the back bumper, you might be OK, at least on gentle roads.
The whole problem is leverage. The 200lb rating is intended for trailers. When a car-trailer combo hits a big bump and bounces, the strain on the hitch will be over 200lbs momentarily, but then the big hit gets transferred to the trailer wheels. When a car-scooter combo hits the same bump, the strain can't be relieved into another axle, so the car has to absorb all that extra strain. If the center of mass is too far behind the car, normal bouncing around will allow the hitch to bend the "frame." I put the word frame in quotes because the Outback is of unitized construction without a true frame, so it's really the whole rear body that will deform. At some point the rear quarters crinkle and the back doors won't close anymore, and it's nearly impossible to repair once that happens.
Be careful and ask the installer if they've done Outbacks before.
If you have ANY option for a lighter weight scooter, driving an Outback is a good reason to take that option.
Also- does that 170lb include the mount/hoist/platform etc or is that the scooter only? Be careful!