Pros
If you are money hungry, then this is a good place for you.
Cons
So many, where to start? Should be ranked at negative stars. Employees have no technical knowledge of basic science, disorganised, work only for the annual meeting, no concrete actions that are sustainable, arrogance is a trait one needs to survive there, they hire staff with inflated salaries and benefits who do no work, then hire consultants to spoon feed them. Something as basic as a donor database, or who the donors are, is not something the staff know. Officers at junior or senior level have no idea on what projects to design, not even basic concepts, let alone the details of a real project proposal. The staff want to show their power and they are managing the projects, but actually they cannot manage, have no substantial inputs to give to contractors, and communicate in piecemeal fashion. It is one big chaos. The right hand doesn't know what the left hand is doing. UNEP gets loads of funding from donors which are mostly wasted on salaries and administrative costs. Meanwhile, civil societies that work much harder with better results and positive impacts to society have a tough time getting funding. It is highly doubtful that UNEP knows the difference between an elephant and a bar of soap.