> Inline functions are still fine though for writing zero-cost helpers, and I don't think C# has them (there's an <AggressiveInlining> attribute but IIRC it's just a hint, the compiler isn't required to actually obey).

In F#, you cannot disable inlining of small functions. It does IL source code inlining, like copy-paste, not machine code inlining. That increases generated dll/exe size a lot for generics. And sometimes you do not want to do that because it's worse for performance. There is an issue for that: https://github.com/fsharp/fslang-suggestions/issues/838

In C#, the AggressiveInlining attribute works well, in predictable manner. Non-inlineable cases are well known (`throw\switch\fixed\try..catch\calling delegates` and some more https://github.com/dotnet/runtime/blob/master/src/coreclr/ji...).

> because it supports directly inlining IL instructions intermixed with regular code

This works for very simple things only. And using InlineIL.Fody, Sigil, raw IL.Emit or just raw IL code as text is not that more difficult if you already know IL.

> you _can_ write F# that's about as fast as non-unsafe C#

I tried this several times. It always ends with fighting the compiler more than just rewriting in C#, even without the `unsafe` keyword. In most cases due to generated IL that I cannot control from F#.