Other people have listed issues in here like compiler crashes from valid code, gotta spend time unwinding changes until it works and write the code differently.

Simple things like "a + b +...+ n" cause compile times to balloon exponentially, which means for arrays you end up refactoring concatenation of immutable arrays into a mutable array you call appendContentsOf on a bunch of times.

SourceKit on the surface sounds great, but in truth it's a simple text parser that doesn't understand type aliases for example.

The compiler _really_ struggles with anything beyond very simple generics, first the type inference starts to fail and then it'll refuse to compile until you break the code apart and add type ascriptions.

The compile errors are often absolute nonsense, pointing nowhere near the actual errors or are like the Magic 8-ball coming up "ask again later".

The project I'm working on has been a mix of Swift and JavaScript, we're happily increasing the ratio in favour of the latter because of all the pain we've had with Swift. I personally don't even like JavaScript but it's not even a contest between the two.