has it ever been criticised?
Yes, it received criticism on the grounds of secrecy surrounding the sources of its data
IUCN Red List
In 1997, the IUCN Red List received criticism on the grounds of secrecy (or at least poor documentation) surrounding the sources of its data. 
how many black stripes does a zebra have UNANSWERABLE what is this animal? They  are African equines with distinctive black-and-white striped coats who are its predators? UNANSWERABLE what is its scientific name? Equus quagga how do they communicate with each other? Yes- by rubbing and sniffing their noses followed by rubbing their cheeks, moving their noses along their bodies and sniffing each other's genitals how long do they survive when they are captured? UNANSWERABLE is it under threat of extinction? Yes-  the IUCN Red List of mammals lists the Grévy's zebra as endangered, the mountain zebra as vulnerable and the plains zebra as near-threatened what is this list? The International Union for Conservation of Nature Red List of Threatened Species  is the world's most comprehensive inventory of the global conservation status of biological species what are its different versions? There have been a number of versions, dating from 1991 when was this system of lists introduced? 1964
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IUCN Red List [SEP] Criticism
In 1997, the IUCN Red List received criticism on the grounds of secrecy (or at least poor documentation) surrounding the sources of its data. These allegations have led to efforts by the IUCN to improve its documentation and data quality, and to include peer reviews of taxa on the Red List. The list is also open to petitions against its classifications, on the basis of documentation or criteria. It has been suggested that the IUCN Red List and similar works are prone to misuse by governments and other groups that draw possibly inappropriate conclusions on the state of the environment or to affect exploitation of natural resources.