Hi Iavor!
I work with/on GHC for iOS daily and have been using it on a large near-production project for ~2 years without trouble. We released binaries for iOS compilers as part of 7.8:
along with these toolchain scripts
and Cabal works well with it too.
I've recently completed a patchset for LLVM and GHC to add ARM64 support that I'll be submitting soon.
As far as building GHC for iOS, all of the patches are in mainline except for an out-of-date libffi, and some tweaks to the bindist system when generating redistributable binaries.
The only major missing piece is Template Haskell support.
Best
Luke