Is EF Core missing anything in .NET Standard vs .NET Core library?

For a class library, the target framework simply specifies a compatibility layer. The ultimate functionality comes from the target framework of the project that references the class library.

For example, let's say that you create a .NET Standard library, and then you include that in a .NET Framework project. Everything you do happens on .NET Framework, and for all intents and purposes you could've targeted .NET Framework from your class library as well. The choice of .NET Standard simply says that you're going to use APIs from some .NET Standard-compliant target. That's all. As a result, the functionality of something like EF Core, ultimately depends on what you drop the library into, not what framework your library targets.

As to whether to choose .NET Standard or .NET Core as the target framework for your library, simply: use .NET Standard unless you have a good reason not to. If you target .NET Standard, you can drop it into any project that targets any framework that is compatible with the version of .NET Standard your library targets. If you choose .NET Core, you can only use the library in .NET Core projects.

Things get a little murkier when you start looking at .NET Core 3.0, though. You can technically target .NET Standard 2.1 and get all the goodness of .NET Core 3.0, but nothing but .NET Core 3.0 actually supports .NET Standard 2.1. You can target .NET Standard 2.0, but then you're stuck with .NET Core 2.2. In other words, if you want to use .NET Core 3.0, right now, there's no functional different between choosing .NET Standard 2.1 or .NET Core 3.0 as the target framework of your library. In either case, it will only work in .NET Core 3.0 projects.

Adding even more confusion, Microsoft is moving the next version of everything under the .NET 5 umbrella. What becomes of .NET Standard at that point hasn't been discussed, by I'd imagine it largely goes away. Once there's a .NET 5, any project can target .NET 5 and be used by any other project that targets .NET 5. That's still a ways off, though. The reason I bring it up, is that if you want to use .NET Core 3.0 today, there's not much point in targeting anything but .NET Core 3.0. Most of the stuff in .NET Standard 2.1 is not back-portable to things like .NET Framework and Unity, so those will never be on .NET Standard 2.1. The path forward for those frameworks is making them .NET Core compatible, to eventually end up with .NET Core 4.0, which Microsoft will call .NET 5.

Long and short, the best performance and features are with .NET Core 3.0, as long as you can go 100% .NET Core 3.0, then that's what you should target. If you need to use your library in something like an Xamarin or .NET Framework app, then stick with .NET Standard 2.0.