Spore-based reproduction incompatible with Ork spores. Whichever set of spores is more numerous survives. So they absolutely HATE the Orks, who literally prevent them from reproducing. Hence the tolerance.
this
also this
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The Xenos rely on
Spores to propagate themselves, much like the dreaded Orkoid menace. This renders their respective biospheres incompatible and is cause for a fundamental loathing for each and every part of the greenskin tide. The Chapter has found no reason to assume their spores won't interfere with Orkish reproduction on mutually infested worlds, but given that the innate savagery of the Orkz seems to extend to even their microbial "offspring," it's doubtful the Xenos would eradicate them without a substantial numerical advantage. However, if a planet was stripped clean of Orkz and colonized by these alien vermin, who can say what would happen?
Like any other species, evolution has endowed the Xenos with strengths of its own, and commensurate weaknesses. Not like humanity, which is perfect in every way. This is known because the Emperor, greatest of humanity, mentioned this in the Lectitio Divinitatus.
What is the Xenos foremost strength?
6
The Xenos are
Iron-Willed, knowing no fear and ignoring pain that would leave the average human in agony. This is clearly evidence of severe mental deficiency but perhaps could be harnessed to the Astartes' ends, once their stubborness has been ironed out. Conventional whips aren't likely to be effective.
9
The Xenos bristle with
Natural Weapons, hideous and unnatural, the source of doom for the foolishly trusting and those unwilling or unable to fight well alike. What form they take may vary, from alien poisons to outright thorns and viler things, but unless they are appropriately clipped, they are never unarmed and for that reason, should never go unwatched.