Why should use NuGet for references
I worked on a task to review references on all projects of a product. I saw many points to be improved and made alot of changes to make the code cleaner, easier to maintain, having better architect by applying abstraction, convert third-party dependencies to first-party... This is not only a pure coding task but also it requires code review & analysis, understanding on abstraction, architecture, design patterns and best practices.
The product has a few solution files and each one contains from a few to dozens of projects. There are different ways of referencing between project to project at different layers and also project with external libraries.
Internal project references
There are some projects on Foundation layer and all dlls are copied to a folder "depedencies/internal". If needed, a project refnerences to the dll under this folder.
External dependency references
Some external libraries are copied to a folder "dependencies/external", a project will reference to the dll under this folder. Some other dependencies will be referenced by using nuget packages.
With these ways of managing dependencies, upgrade to a new version of any library is hard (both internal & external), because dependency's version number is specified in target project.