In my experience porting from .NET Framework to .NET Core isn't that difficult per se -- it is the frameworks that we are stuck on that cause the difficulty, namely if you have a large project using ASP.NET pages and EntityFramework classic, which do not have compatible .net core versions really. The .net core replacements are objectively better, but have significant conceptual and API differences that prevent a simple move.

There are some community projects that try to fill this void by porting entityframework classic to .net core for example though.