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I've had conversations on the subject with park rangers on trails I ride in PA and MD. No warnings or citations though. In both states, the rangers say they look for reckless riders rather than check bike specs. It would be more of an issue if you were involved in an injury accident. The lawyers will certainly bring up the point if you ride an illegal bike.
Keep in mind, it isn't so much the motor wattage but rather how it is applied. A 500 watt motor on a mid drive bike with a good range of gears will easily climb steep grades. 500 watts on a hub drive not so much.
If you don't already own a bike, try test riding both types. In an area with many hills, a mid drive would be the best choice.