Most domain registrars require providing identity details. Even if these details are private, a single leak or a config mistake on this domain will expose your real identity, tied to all aliases. With an alias service or a shared email provider you don't have this risk as you don't have to provide your real-life identity.

So while it's tempting to use one random alias (h3hj4gjh234@yourdomain.com) for a high-risk service and another alias for a critical service (github@yourdomain.com), these aliases are easily identifiable as belonging to the same person.