i decided to make an original system because no other system could offer me exactly what i wanted; long, involved combats with quickly-resolved rounds, the ability to express character personality in a very specific mechanical way, the rewarding of playing according to narrative themes by consensus. lots of games had one or a few but not all.
the biggest challenge is making it do exactly what you want is difficult, and you have no idea if it works. or if it only works for you. you're wading through unexplored territory, and no amount of conventional wisdom can prepare you for where you need to go.
the biggest advantage is that you are wading through unexplored territory; conventional wisdom does not apply. it's freeing because nobody can tell you what you should be doing.