"Unep, the UN Development Programme, the children's fund Unicef, all suffer as well from the way in which rich countries, the US in particular, use their purse strings to manipulate the UN.
The UN Environment Programme (Unep) is underfunded, understaffed, and often blamed for failing to cure the world's environmental ills.
Many sober voices, scientists and politicans as well as environment campaigners, say the Earth's plight is serious and worsening.
They point to population growth, the depletion of resources, pollution, the water crisis, and above all to the prospect of climate change.
They wish that Unep were rich and powerful enough to reverse the trend. But, increasingly, they recognise that the agency, like so much of the UN, promises far more than it can deliver.
The blame for that, critics say, lies not with the UN but with those who shape the world in which it has to operate.
Andrew Simms, of the New Economic Foundation, is one of the authors of a report on the working of the UN, entitled It's Democracy, Stupid.
He told BBC News Online: "Unep is in ways a symptom of the problems that beset the whole UN system.
"It was limited for years in what it could do by the politics of the Cold War.
Manipulated
"Now it finds itself up against the policies of the International Monetary Fund and the World Bank.
"What it can do will always be bitty and piecemeal, all its projects swimming against a hostile tide.
"Unep, the UN Development Programme, the children's fund Unicef, all suffer as well from the way in which rich countries, the US in particular, use their purse strings to manipulate the UN.
"They're the agencies that have made the running on environmentally-friendly human development, but they're up against the hostility of unaccountable institutions.
"Even so, they've done a lot more than anyone else on issues like climate change and species loss."
Unep was widely praised for its publication of the Global Environment Outlook 2000, an exhaustive appraisal of the state of the planet at the start of the millennium.
It is planning a similar report in 2002, reviewing progress since the 1992 Earth Summit in Rio de Janeiro.
Much of the UN's work on the environment is done, not by Unep directly, but by commissions and agencies created for specific tasks.
The UN climate change convention is the lead player on efforts to confront global warming, and Unep's role there is partly to mediate between countries which want to tax their way out of trouble, for example, and those which want to buy and sell permits to pollute.
Criticisms
Sometimes, as with the biosafety protocol, it is in effect shouldered out of the way by political heavyweights like the World Trade Organisation.
One supporter of Unep, and of the UN system at large, does have criticisms.
"The entire UN must be much stronger in working to create trust between the governments of North and South", he told BBC News Online.
"And there is too little integration between UN agencies with overlapping jobs - between Unep and the Development Programme, for example."
The present head of Unep is Dr Klaus Toepfer, the former German environment minister. He has little doubt where the greatest problems lie.
Poverty
Last year, he said the developed countries must cut their use of natural resources by 90% to give the rest of the world a chance of emerging from poverty.
And he told an audience in Japan on 3 September that the lion's share of environmental degradation stemmed from the consumptive behaviour of the richest fifth and the desperation of the poorest fifth of the world's people.
Unep has a voice, and is prepared to use it.