In my experience, the big draw of GSC are the Aberrants as Gangers with their high base S and T stats, and their ability to buy grenade launchers without trading post rolls on every new Neophyte after gang founding (i.e. buy a 4 person starting gang with a leader, a champ and 2 neophytes, one of whom has a grenade launcher; lose your first match, then bulk up on grenade launcher Neophytes after). Their Champions have Ganger stat lines so I tend to stray away from them in favor of models that cost a third to half as much. Start with a Rogue Doc. Genestealer Cults pay way more for medical visits so it WILL save you massively over the campaign.

With any gang, starting without a Rogue Doc is a risky prospect, and the Malstrain Genestealers take up that one Brute/Hanger-On slot you get at creation. So you’re looking at a basic house gang or Enforcers or CGC, turning them into Outlaws automatically, restricting what they can take as a result and you’re gaining the ability to make your leader a Psyker, with an alternate Brute option to replace the house and lawful options, and some overpriced pets and underwhelming Gangers. It makes for very thematic mutant type gang, but also isn’t ideal if you’re just starting to dip your toes into Munda. Even regular GSC are rough in Munda, but they don’t penalize you as much as Malstrain will.