Hds are no more dificult to understand than a MD with 3 major differences as far as the tuning scheme goes. Asuming a 15/14 hd both the hd and md asuming DAd are both 3 octave insterments and both play the same exact notes with one difference. That has to do with the half frets on a md but that is also easy to deal with. Both are diationic in nature but with a HD the octave is split in the middle over the middle (treb) bridge with the low 4 notes on one side and the other 4 on the other. The same relationship holds between the right side of the treb and the bridge (Bass) to the right. What this gives is a diference of 5 notes across the bridge on every course. If you know anything about chords this gives both the low note and high note of a chord on every course. You have 2/3 of a chord and all you gotta do is add in that middle note, located the third string up on the right, if you preffer doin melody.
Secondly a md is more suited to doing harmony than a HD because theotectly you can get a different chord every strum if you chose to do so. Its also easy to do just pure melody or a mixture of both also. A Hd by nature is more suited to do melody on because you have only 2 hands and a true 3 note chord on a single beat is impossible because youre short that third hand.
Thats not to say you cant do melody on one, you just have to resort to doing chords by cheating a bit and relying on the substain to get the chord sound. The up side of that is any chord is just as easy as another because youre not limited by how far you can spread the fingers. There is all sorts of tricks you can use to get a chord sound but that is a subject all its own.
While a Hd is diationicly tuned just like a MD the 3rd difference is you have many keys avalable without retuning or capoing or other tricks. Just start at the right marker for the desired key. Granted, the mechanics of playing the two are tottaly different but if you understand how one works, the other one works simular. Music theory such as timing, actual notes and so forth is exactly the same for every insterment so theres no relearning as far as that goes. In fact a md tab works quite well on a HD too with a minor adjustment. There are no frets on a HD as each course (left side and right side of a bridge on the same course counts as 2 notes) so each note is essencally an open tuning. In essence just add 1 to the numbers on a md tab and count courses starting from the approate key marker.
The beauty of how a HD is set up note wise is the layout is actually one big music "cheat sheet" once you learn how to read it. Its super easy to play any tune you know in one key in just about any key on a Hd without even knowing the notes of the other key too. Hds are what one could call a pattern insterment. Once you learn the pattern for a chord, that same pattern holds almost everywhere. The exception is where you "run off" the top of a HD. All that changes is you gotta know where to start the pattern and thats the naming note of the chord.