Note that, for most web users, they will see "somepart.domain.com" as just another part of "domain.com", and so if there is something on "somepart.domain.com" they disagree with (say, adult content), they will blame "domain.com" and complain to you, and/or your registrar, about said content.
That depends upon your definition of "safe".
Note that, for most web users, they will see "somepart.domain.com" as just another part of "domain.com", and so if there is something on "somepart.domain.com" they disagree with (say, adult content), they will blame "domain.com" and complain to you, and/or your registrar, about said content.