The Race X's natural innate ability that classifies them is their ability to instantly and precisely teleport. Something that takes the Combine an entire building sized power supply and a massive structure to accomplish, and even then it's incredibly slow to charge and fire.

The Race X are capable of breeding the Shock Roaches, which are extremely dangerous to the point that they can even reduce someone to meaty chunks if hit right. All they have to do is pick up the creature they want to use and it's a weapon, while the Combine have to pick apart their prey for god knows how long in order to get any use out of them, and even then they are usually more fragile than Race X soldiers.

Race X enemies like the Voltigore use extremely powerful electric shocks that will reduce anything in the vicinity to a pasty smear on the environment, possibly even capable of breaching the armor of Combine Synths and reducing them to mush.

The Pit Worm and Gene Worm can take a shit load of punishment, are immune to every type of traditional gunfire, and are capable of shredding anything with absolute ease.

What I'm saying is, the Race X are more than capable of kicking the Combine's ass, but due to circumstances, the Race X are forced/threatened to stay in line with the G-Man and his Employers.

Want more of an explanation?

Well, you shall see what this is for in Half Life: Adrian!

Game by TQC, Half Life by VALVE, Race X and Adrian Shephard kinda belong to Gearbox but not Greasy Randy.