I prefer monorepo for tightly coupled systems (like a game server and client), and multi repo for multiple, loosely coupled systems (like a web API and mobile app). The main problem with monorepos is that without the resources to build proper tooling, you end up with useless commit logs and losing track of context while working, but once you're on a larger team the advantages may be better. However, if you have multiple teams working on different, a multi repo might be better; for example, if you have a team for your API and a different team for your website, then multiple repos might be better depending on how coupled they are.

Either way, it doesn't really matter that much. In the end, it's mostly just a personal preference.