On my non-folding 1952 Johnson JW-10 3 HP I broke the shear pin once on a sunken stump. Although the Johnson and Evinrude weedless 3 Hp motors have a rubber hub in the propellers to absorb most shocks, if you hit something hard enough the shear pin will break to save the rest of your drive train. Otherwise these little motors are bombproof, just keeep them fed with fresh gas and lube and always close the fuel shutoff valve then run the motor dry when finished for the day to reduce the risk of gumming up the carb.
I have heard that the folding shafts are more fragile than the non-folding ones, so the shear pin may not be enough to prevent major damage, such as at the hinge point on the leg. Nowadays I usually use my folding 1966 Ducktwin at the duck marsh, so I slow right down if there are stumps or sunken logs. After all, it's not a race.......