As a race, Hybrids start their service life very poorly regarding physical stats but with high psionic skills. But as they mature, they can become incredibly powerful, making them good long-term investments.
The problem is keeping them alive that long whilst still exercising their powers. In order to use their psionic powers hybrids need to be on line of sight with its target which frequently exposes them to danger. Training a hybrid's physical stats might give them better survival odds, along with the use of armor and shielding but it will take longer than developing their psionic skills and it isn't possible to do both at the same time. Plus a hybrid will almost never be able to achieve the physical levels of androids or veteran humans.
This lack of combat skills and their low numbers available for recruitment will work against deployment of purely hybrid squads since any combat deaths will be hard to replace. Hybrids work better when they are integrated into teams composed of all races, where they can rely on the protection of their fellow agents to properly use their psionic skills.