I like the Mad Max-esque theme to them. Also, as an American, I kind of like the underdog narrative that rises up to overthrow the oppressors (don't confuse that with SJW trust-fund babies and the like who have a shallow sense of justice). Ironically, for some reason I don't really like the Tyrandis though; so, the GSC army I'm building (my second army after Astra Militarum) has a kind of independent flavor to them. I know that there are theories that some of the wildlife on Catachan is distantly related to the Tyranids, perhaps splinters of a hive fleet that arrived on Catachan long ago and lost touch with the hive mind. So my GSC is kind of an independent group from the rest of the Genestealer Cults who are looking to the stars for their overlords. Instead, my GSC fights for the jungles of Catachan, striving for the time when the jungle patriarch will gain dominion over the entire planet and drive out all foreign species, whether that be the Imperium, Chaos, Xenos, or even other Tyranid hive fleets. It makes a cool back story for my Catachan Brood Brothers, men and women born and raised on Catachan who fought the planet's will their whole lives but have now come to see the truth behind nature's plan. Having joined the Catachan GSC they come to realize that for the first time, they find themselves fighting with Catachan and not against it.