We use a mono repo for a new cloud based solution. So far it's been really great.
The shared projects are all in one place so we don't have to kick things out to a package manager just to pull them back in.
We use filters in azure pipelines so things only get built if they or dependent projects get changed.
It makes big changes that span multiple projects effortless to implement.
Also running a local deployment is as easy as hitting run in the ide.
So far no problems at all.
A certain world event being a 3rd party piece of software having a bad update.