The advantages are clear
There's no need to re-enter your personal information on a website other than the one you normally use. You forget to remember your password, which seems like a problem these days given the number of systems you use.
Among its disadvantages
The level of privacy compromise is lower, since each site requires permission to access your social network, which makes you more vulnerable to information theft by these networks. This doesn't just involve your personal information that you've deliberately entered, but also your likes, preferences, friend lists, and algorithmic relationships that the social network creates based on your behavior in its system.
The win-win situation of the non-contractual relationship established between this website and the social network means that your preferences are tracked more effectively and that you are the target of advertising campaigns, which has become increasingly difficult. On the one hand, the website gains greater conversion rates by making registration easier for the user-customer, and the company makes its users more dependent on it.
What happens if your social network no longer exists? You will no longer be able to log in to the website because the login is determined by the social network session. That is, if you delete your Facebook account or your account is deactivated due to the social network's policy, you automatically lose access to the login-dependent website, and when you have critical services on this second site, this can become problematic.
As a personal recommendation, if privacy policies are not a big deal to you, you can use it for content-based websites (blogs, forums, etc.), but for critical services (electricity, water, video games) where you usually pay subscriptions, conventional registration would be ideal.