One is much simpler than the other. Artifact management is surprisingly complex, it only looks like it works great while you aren't managing many versions and acting more like a monolith, just spread across repos/deploy points.
You already have that pipeline for release/deployment though, don't you? (And it's already hooked up to your SSO or what have you).
> if you have three projects in three checkouts that all have to be coordinated and merged in order so that you don't accidentally break your environment and worry about security and and and....
It's the same as any other library dependency though, which is a completely normal thing to deal with. The cost of separating your pieces enough that you can use different versions of a library in different services is that you can use different versions of a library in different services.
I'm skeptical about microservices as a deployment model, but I'm absolutely convinced that code-level modularisation and independent release cycles for things that deploy independently are worthwhile, at least if your language ecosystem has decent dependency management.
Nuget does provide mechanisms for obsoleting packages, so it's reasonable to enforce that new packages should allow for a few versions worth of backwards compatibility before deprecation and finally pulling the plug.
This sounds like it could work, but then you start to wonder about how you'd get common code into those 12 services. Our answer at the time was nugets, but we operate in a sensitive domain with proprietary code, so public nuget services were a no go. So, we stood up our own goddamn nuget server just so we could distribute our own stuff to ourselves in the most complicated way possible.
Even if you are using all the same language and patterns everywhere, it still will not spare you from all of the accidental complexity that otherwise becomes essential if you break the solution into arbitrary piles.