I think it’s because if you do Angel Cartel missions, you ruin your Minmatar and Gallente standings and become hostile in their regions. (The same way if you run missions for Minmatar you will hunt Angel Cartel and some Serpentis and other pirate NPCs and ruin your standing with Angel Cartel and Serpentis etc.)
Also Angel Cartel pirate agents are not placed in high sec so you have to be prepared to live and mission in a less safe environment.
you have to go to low-sec or null space, and deal with the PVP attacks there
their missions ask you to kill empire navy ships, which ruins your standings with the empires
after you build high standings, NPC pirates will still attack you in belts and at gates, because they are pirates
The rewards, like with any mission, are ISK, LP, and standings. You can get ISK from bounties elsewhere, and standings only help you advance in agent ranks. The LP is the true reward, because you gain access to the pirate faction LP stores, and they have the pirate ships for sale (Machariel, Rattlesnake, Vindicator, etc.). You can make a profit selling these on the market, but you have to have the logistics ships and connections to transport these ships from null or low-sec to Jita.