I'm afraid even the anecodotal evidence is damming enough. In my experience, it's true. I even had my motor glider landed wheels up by someone we let have a go in it. Luckily, the only damage was to the wooden prop and the pilot's wallet as it was a RF4 which the designer had thoughtfully put strakes in the bottom of the fuselage for such an eventuality.
I did a wheels up in my glider once - deliberately. I could only get the UC down half way, so I chose to land sans gear to prevent the doors being ripped off. No damage to the glider, apart from a grass stain. the cause was a camera lens cap jammed in the gear mechanism. We didn't recognise it, so it must have been there many years, probably dislodged from it's resting place by a negative g bunt after a cable break a few launches earlier.