Another misinformation by @fysiks as usual, that doesn't surprise me anymore.
rehlds is an open source project, means that if you download it from github, it's almost guaranteed that no backdoors will be installed, otherwise even amxmodx wouldn't support rehlds.
Github links :
https://github.com/dreamstalker/rehlds
https://github.com/s1lentq/ReGameDLL_CS
Pros of rehlds + regamedll_CS :
Better protection against low exploits that can crash the server from malicious software.
HLDS is currently vulnerable to some of them.
Better stability : no more random crash from the server.
So far for me, no more random kick with SVC_BAD.
Various bugfixes, IE, no more player in the ground (or in the sky) bug, the .wad bug...
Better performances
Maintained by an active community with updates and improvements
Cons of of rehlds + regamedll_CS :
Not official valve binaries, since it's maintained by a github community
Incompatibility with Rage and Orpheus or some modules (CSDM IE)