They're absolutely not as preventable as aiptasia. Even tanks with voracious bristleworm predators have bristleworms. Their populations are heavily controlled and you may never see them, but they're there. Imo they're much more akin to pods where you just end up with them eventually no matter what you do.
Also for really controlling bristleworm pops you need a BIG fish and a truly predatorial fish (read: also usually nibbles hermits, snails, and small shrimp). We're talking triggers, large wrasse, puffers, or hogfish of fish we usually see in the hobby.
What are you feeding and how often? Bristleworms can absolutely be controlled by feeding quantity, and that's how a lot of people with large tanks who want to keep inverts control their bristleworm population.