Is there anything AHK can do that Keyboard Maestro cannot?
Being free is a big one.
Yes, AHK is "lower level" regarding input tracking, so in KM left vs right modifiers aren't properly supported (though there are hackarounds that can make it work)
Then the sripting language is easier for batch editing and various permutations with proper functions and variables (though again KM can be partially hacked around to make it easier)