I’ve never touched a MD, but definitely can see where it and the AR differ and overlap. I’m on the AR mkI, and don’t find any of the engines ‘limiting’, but you definitely need to have a plan in mind on how to use them depending on what you want to achieve.
As far as chromatic stuff and the range you can get out of it, that is somewhat dependent on your own ears. I find even high register stuff can sound good when properly tuned and filtered. the DVCOs have a huge range in line with simpler monosynth like the SH101 and Moog Rogue. Plus using stuff like the cowbell as a synth can really sound nice especially once you start layering a sample behind any of the machines.
Also programming melodic stuff is pretty easy in my opinion. I don’t particularly gel with how something like the Digitone or A4 works, but with the AR you can hold down a trig and tap a pad in chromatic mode to enter the note. This also lets you preview it so that you can get the velocity how you want it.