Classic site collections and modern sites are functionally equivalent in terms of the access controls available. In essence, a modern site is a single-site site collection. One of the most important decisions to make from a security standpoint in either user scenario is whether or not to allow external access. If you block external access at the site collection level, no one in the site (regardless of their access) will be able to share content from that site directly with users outside your Office 365 tenant—a configuration that is useful for proprietary or private business information.