The UPP project began in 2008 and - from my conversations with several people living in favelas - was part of a larger effort to improve living conditions, which it has to an extent; some favelas now have basic sewage systems, for example. However, the police presence was seen as ineffective and the project disingenuous in that it was inspired by the desire to make Rio a stronger attraction to tourists during the World Cup rather than by the concern for the welfare of people living in favelas.
Additionally, the relationship between law enforcement and residents was also quite crooked. The Guardian reports that "80% of Brazilians are afraid of being tortured by their own police force," but the study was conducted by Amnesty International. http://www.theguardian.com/commentisfree/2014/may/18/brazil-...
Last, I can't imagine policing in favelas to be an easy job. When I visited a pacified favela, the police unit walls were lined (50+) with large bullet holes.