[Haskell-cafe] Questions about slow GC with STArray

[FFT]

I've been following with interest the recent discussions on reddit
about the extremely slow hash tables in Haskell compared to F# and
OCaml, and as I understood it, this performance problem is caused by
GC not liking mutable arrays
http://hackage.haskell.org/trac/ghc/ticket/650

It appears from the discussion there that this is more than a simple
bug, but a fundamental difficulty (slow writes vs slow GC trade-off).
What I'm wondering though is how can this be unique to GHC: all arrays
in OCaml and probably F# are mutable (and usually unboxed). How is
this problem addressed there? Why is this supposed to be specific to
boxed arrays only: wouldn't GC have to scan the whole mutable array
whether it's boxed or unboxed?