Multisite lets you run multiple sites from a single WordPress installation. One database, one set of plugins, one update cycle. Sounds efficient, and it can be. But there are scenarios where it creates more problems than it solves.
Right use cases
Universities with department sites. Municipalities with sub-sites per division. Companies with regional landing pages. Everyone shares the theme and core plugins but has their own content.
The advantage is obvious: update a plugin once, all sites get it. User management is centralized. One server to maintain instead of ten.
Wrong use cases
If the sites need entirely different plugins or themes, multisite becomes a bottleneck. A buggy plugin activated network-wide can bring down every site at once.
Hosting gets trickier too. Not all providers fully support multisite, and migrating individual sites out of the network is considerably more complex than moving a regular WP installation. We've been through migration projects that took three times longer than estimated, specifically because it was multisite.
Subdomain or subdirectory?
Subdomain (blog.example.com) if the sites should feel independent. Subdirectory (example.com/blog) if they belong to the same brand. Both work technically, but subdomain requires wildcard DNS and sometimes wildcard SSL.
Our advice
If you're unsure, run separate installations. It's always easier to merge sites later than to break apart a multisite network. We say that after having done both more than once.