"Microsoft's unification of .NET is in reality a process, not something that happened overnight with the release of .NET 5.0 last year."
Sort of, not really, no. Microsoft's "unification" of .NET is more about adding features to .NET Core to make it suitable to replace .NET Framework, Mono or any other .NET runtime. There will not be a point where you can compile .NET Framework code in .NET 6 or 7 or 8 etc. Microsoft has made this clear. They're providing some tools to help you migrate, but it's a big task. Skills transfer very easily between .NET Framework and Core but when it comes to software you need to migrate a lot of things. For web applications MVC Core is very different from the older MVC, the entity framework versions are different (yes, you can use 6, but it doesn't interface with MVC Core so that's also an issue, although one that might be fixed at some point).
The TLDR is that it's not so much a unification is it is a replacement. This is a big change for Unity to support.