I worked on a (failed, explanation follows) project to trial port a large system from .NET Framework 4.6 or 4.8 to I think .NET Core 2.1 or 3.0. I was most annoyed at having to recreate project files, but it was quite straightforward and easy, even for us, where we had complications like numerous sub-projects, some generated code (luckily, I quickly found the correct arcane incantation embedded in some GitHub issue), etc. We also had to rewrite a C# API wrapper for a C library we used for reliable multicast, because our vendor had disappeared, and their wrapper was shipped as a Windows-only Framework-only DLL (and honestly, the code quality was relatively poor, with a lot of needless complexity in their wrapper). But we had already consciously chosen to only use sensible slices of that library’s functionality, so writing our own wrapper was quite trivial, even though I hadn’t used C#’s DllImport stuff before. It was easy to port the slice needed for most publishers first, before having to port the slice for receivers, etc.

All in all, I’d say the port for our large suite of server apps, with lots of different CPU/memory bound and network I/O bound apps, it was quite straightforward and easy.

What killed it in the end for us, was that our testing showed our CPU/memory bound apps ran 0.9x as fast on Linux .NET Core 3.0 vs. Windows .NET Framework 4.6/4.8 OR .NET Core 3.0, using near-identical hardware. The Linux port just wasn’t there yet, so we decided not to go through with any port to .NET Core for any part of our system, since Linux would have been the main benefit (we were using fancy network cards and stacks, and the Linux support for those was far more advanced and reliable compared to Windows, where multicast groups would spontaneously fail to be joined, etc.).

Another elephant in the room, is that GUI support on .NET Core was limited at the time, so we may not have been able to do a Big Bang for that reason also. We had a few WPF apps, and some Forms stuff, but I have no clue about what .NET Core support was/is for that stuff, as I rarely worked on GUI things (I would create a GUI as a necessary part of an experiment and hand it off to more experience people, or modify/simplify GUI logic as part of my evolution of our server stack, but that’s all).

So obviously YMMV and it seemingly does vary, but my experience on server apps that weren’t using lots of Windows/MS specific technologies (I’m not a fan of them generally), our porting was really quite easy. Still a failure though :-)