Multisite is not hard to install. It is hard to change your mind afterwards, and both of the decisions that trap you get made in the first ten minutes.
The first one is subdomain versus subdirectory. Subdirectory looks tidier and needs no DNS work, but it eats your URL space forever, so a site at /shop means the main site can never have a page at /shop. Subdomain needs a wildcard A record and a wildcard cert, which most decent hosts will do these days, and it is the only sane pick if the sites are seperate brands. Switching later is possible but it means a search replace across every table plus redirects, and I have never seen that go cleanly.
Second is whether you actually need domain mapping. Since 4.5 it is in core, no plugin required, you set the domain on the site in Network Admin and point the DNS. What nobody mentions is the certificate side. Every mapped domain needs its own cert, so twelve client domains means twelve renewals, and if the host does not automate that you have signed yourself up for a monthly chore forever.
Two smaller things worth getting right at install time: 1. put define('WP_ALLOW_MULTISITE', true) into wp-config before you run the network setup rather than fighting the admin screens later, 2. decide now whether site admins get any plugin activation rights at all, because taking that away after six months of them clicking around is a conversation nobody enjoys.
Nearly everything else in multisite can be undone later. Those two cannot.
Source: r/WordPressToolkit · by /u/Bright_DD