I just finished John Keegan's book on The Second World War, and he makes essentially the same point you do -- for all the super-human bravery of the people involved with this work, it was mostly difficult or impossible to operate in Nazi-occupied Europe. Many operatives were parachuted in to Germans who were waiting for them when they landed, because the clandestine groups had been so thoroughly penetrated.
The British were able to do something similar in England -- the Nazis thought they had operatives in the UK, but all of them had been discovered, and the "information" they provided was bogus.
Keegan says the far less glamorous work of the chairborne cryptoanalyst, who could break codes, had a far great impact on the war.
The msot exciting action superspy Kim Philby ever accomplished was sneaking out of England before he was arrested.