Cronenberg was pegged early on as a horror director, albeit a director of horror films that didn't fit well in the genre as it had been established up to that point. A lot of this was due to his interest in particularly visceral horror, especially that which affects the body from within rather than from without. Even before I'd seen anything he'd directed, I knew him by reputation as a director of horror in which the monsters aren't any kind of external force but rather come from within, in the most literal way. (That the title of his first feature film is, well, They Came From Within is telling.)
This unorthodox approach to the genre is buttressed by Cronenberg's total lack of interest in traditional monsters, which gives him quite a lot of freedom from audience expectation. He uses this elbowroom effectively, not so much to build tension or work towards scares as to keep us just as in the dark as his characters regarding where their mutations and physical reconfigurations will take them. Though we can be sure that it'll be somewhere revoltingly squishy and biological, we're never quite sure where a given story will end up, since we've got no map of previous similar experiences to compare it to. This liberty is one of his most powerful tools, a good narrative trick to involve his audience in the story, since he's not adept at creating characters who are nuanced enough for us to identify with.
That his monsters are just as human as their victims suits this inversion of the normal horror structure of an external threat challenging a community or individuals. Cronenberg's insistence on portraying his characters as well-rounded, if not deep, also means that there aren't any faultless protagonists or entirely diabolical antagonists. The conflicts here aren't those of entropy versus order as per the horror standard, but rather evolution versus stasis, which makes for a much more ambiguous arena for his ideas to do battle in.
The Brood, then, is a bit of an outlier in his body of work. The protagonist experiences only external threats and doesn't experience any kind of transfiguration as a result of encountering those threats. The primary antagonist isn't show going through much of a transformation, either, though the way that the movie gradually reveals what she's been all along acts as a kind of narrative surrogate for such a transoformation. Mostly, though, Samantha Eggar's portrayal as Nola leaves quite a bit to be desired, as she's an antagonist who can only be empathized with on the most abstract of levels, and not through the fault of being rendered inhuman due to her transformation. Though her primary motivation of reuniting her family and convincing them to love her unconditionally is easy to relate to, Eggar chews through the role with so much vehemence that it's hard to imagine her as anything other than a murderous shrew.
Cronenberg is certainly a director whose main strength is his ideas, but his execution often isn't up to the task of making those ideas come to life. Here, that's most evident in the brood themselves, which should be a terrifying personification of blind rage but instead end up being nothing more than attack-midgets armed with a variety of improvised weapons. Though they act as a fine visual metaphor for the way that domestic violence is often passed down through a generational cycle, particularly in the way that their dress mimics that of Frank and Nola's daughter, there's nothing particularly frightening about them in a more immediate fashion.
The presence of the brood is also made problematic by the way in which the thematic conflict of evolution versus stasis is presented as so unequivocally in favor of stasis as a favorable force. Though Dr. Raglan's other patients don't commit murder by proxy or lapse into messianic delusions, they also aren't shown to be able to produce anything other than self-defilement through the application of Raglan's psychoplasmic techniques, and there isn't a single case in the film of those techniques producing any results in the realm of psychiatric therapy. All of his patients are demonstrably just as crazy as they were before they began treatment, with frequent Cronenberg collaborator Robert Silverman portraying an especially zany/fun version of the demented character he's often called upon to play.
A shift toward more immediate altruism near the end of the film sheds more light onto Dr. Raglan's character, which had seemed pretty one-dimensionally Evil Scientist up until that point, but the arc of the story finds itself in a strange cul-de-sac after revealing just how strongly it presents evolution as a threatening force. Though the pattern of familial abuse is shown to have originated prior to the advent of psychoplasmics, use of that radical superscience is shown to be no cure for that traditional social ill. In fact, the very nuclear family unit that's shown to be the source of Nola's rage in the first place is only reinforced by the ending of the film, with the final shot serving as a reminder that this is a conflict which seemingly has no resolution. That lack of easy answers, or perhaps any answers, is also emblematic of Cronenberg's work, in which conflicts which may initially appear to be resolved are shown to be fractal in nature, whose resolutions are elusive at best.
A special note needs to be made about the trailer for the film, which is amazingly hyperbolic. As noted previously, there wasn't really anything scary so much as thought-provoking about The Brood, but the trailer takes a different tack entirely. I'm fine with this kind of misleading approach, as "The movie that will make you consider whether or not domestic abuse is unavoidably hereditary" doesn't make for much of a dramatic voiceover. But, still, the wannabe-ominous narration intoning how the film will send my brain reeling from the most terrifying images of terrible terror ever recorded, while onscreen we see nothing more than a box of Shreddies being knocked onto the kitchen floor, was just too rich to believe. I'm glad I didn't watch the trailer before the movie itself, but I highly recommend checking it out after you watch the feature as a quick but marvelously potent shot of unintentional humor.