In theory, the idea of generating so much money for conservation is fantastic. But it is difficult to say if money actually goes back to communities or conservation. This can be very hard to monitor and there is a current lack of data on this. In poor countries with weak governance there is a lot of opportunity for corruption.
Although there are success stories (such as the white rhino), this is not the case for all species. Lions in particular are declining all over Africa and several studies, for example in Tanzania, have shown trophy hunting to be the main cause. Management of lion trophy hunting may need further research with some factors unique to lions. For example, the removal of males may cause high turnover within prides resulting in more infanticide (new males kill infants when joining a pride to remove offspring that do not share their genetic make-up and to bring the females into oestrus again more quickly).
Careful management is also easier said than done. It can be hard to define quotas, especially when exact numbers of existing animals may not be known. Quotas that seem conservative may actually be detrimental to the population, for example, in species where the males provide paternal care.