They're really not comparable at all. Executor is a 68k emulator + APIs for OS7; the tough part there is doing the emulation properly, and then just implementing all the APIs. Darling is just a binary loader + APIs; the loader is trivial but there are a bajillion APIs. Probably 3-4 orders of magnitude more complexity in Darling, simply due to how much is in OS X.
I do think they're on the right track, though. Hoping to lend a hand when I get some free time.