Will my site be less secure if I introduce a subdomain-based multisite network?

I'm about to expand my existing website by introducing a way for visitors to create their own store.

I'd like to create the network at domain.com so users can create stores with a close relationship to the brand (wookiecookies.domain.com), but I wonder if this will result in a website susceptible to hacking.

I've noticed websites like Shopify designate a different domain specifically dedicated to user activity (myshopify.com), so when they create a store it results in wookiecookies.myshopify.com by default.

Because of this, I'm inclined to create an entirely different domain where users can create their own store and upload stuff ... but at the same time, I'd love to get their store as close to the actual brand as possible, creating a more seamless experience.

What do you think guys -- am I just over thinking it?