They virtually can't pinch flat because the tube doesn't get pinched against the rim. They will puncture from glass, thorns etc., like other tires. How suceptable any tire , clincher or tubular, is to punctures depends more on the weight and type of material than whether its a clincher or a tubular. Having ridden tubulars for 20+ years, there is no way you can tell me a 200g silk tubular is flat proof.