Old-canon for Ork-Hybrids was that they were seen as Oddboyz of their own sort and kept on the edges of society like Weirdboyz and Burnaboyz. At least until enough hybrid spores were around and Orks were infected and enthralled that the cult outnumbered the normal boyz.
As of GCs they can apparently grow to be great enough in number to build and crew Gargants, which is impressive, and be powerful enough to fight the uninfected Orks and Imperial Guard at the same time.
Important to note that at least in the current canon cults go in cycles, each composed of 5 generations of hybrids and the infected humans who start the chain.
Patriarch infects a victim, who becomes a thrall, then it goes:
Thrall>Gen 1>Gen 2>Gen 3>Gen 4>Purestrains
then it loops back around as the Purestrains make more thralls and the Patriarch evolves into a giant monster as he soaks in the accumulated psychic might of the cult.
Most cults apparently go through 4 or 5 cycles before rebelling, though they do a lot of smaller scale fighting before that point. Killing gangs, overtaking mining bases and factories, then just working in them and paying their taxes. Hybrids usually make better citizens than actual humans do.
It's also impossible to tell on sight who's a thrall, they're still the original race, they just make alien babies and are part of the broodmind, which compels them to worship the Patriarch as a god and the genestealers as saints.
It's actually kind of tragic when the Tyranids arrive and the 'stealers turn on their families. The cultists at least genuinely feel a loving bond with their purestrain family members, and it's implied they might feel something back since even the early gens who're more 'stealer than man have feelings. Purestrains often get painted with religious symbols by their mothers and fathers before going into battle and for special occasions.
There's a little story snippet in the codex where a 4th gen is trying to lead his squad to help the Patriarch, who is fighting alongside the forces of a Hive Fleet at this point, and is headed off by his own purestrain son, and he of course is happy to see his kid, until it rips his head off in the blink of an eye and slaughters his unit because it's part of the Hive Mind now rather than the Brood Mind.