Larger labour providers were particularly frustrated that the Act lumped them in with unregulated field gangs, some of whom they believe remain out of the reach of the licensing process.
One labour provider said: "To be put in the same frame as these guys who'd had been up to this nonsense in Morecambe Bay wasn't really pleasing. You're an agency, you're doing some work in the food industry - (therefore) you're a gangmaster."
He added: "The highly unregulated labour supply still goes on. It won't make any difference to them because they just carry on as they are ... So my people have to do an awful lot more to comply with the regulations and the unregulated supplier doesn't have to."
Gangmasters spoken to by the researchers could also cite cases of "double standards", and most claimed to know of a local "Janus-faced" operator. The concern was that the GLA was unintentionally "licensing crooks", the report said.
One operator told researchers: "Without mentioning any names, I can see when looking on the list of licensed people (some that) I know have never paid their tax ... They should never have been licensed in the first place."
The study concluded that the GLA needed to address such concerns over the next year.
Despite the criticisms, the report found a consensus that licensing had formalised a large segment of labour providers, changing the habits of many middle-ground operators. "It's good," one grower said. "It's taken some of them out. Before the register we saw a lot of people saying 'we're going to go for it', then three weeks before they were saying 'we won't bother'."
The study found contracts and workers' conditions are now more transparent than before, and health and safety taken more seriously.