I agree, I can write a bad song that fits any style of metal without much effort.

There is no way to quantify difficulty of composing a good song within a genre, nor is there a non-arbitrary way to quantify how good a song is. Is it harder to compose a good, memorable, and original song using three chords than to compose a highly technical progressive song? Attempting to quantify complexity as what makes music good results in a few musicians playing little more than their warm-up exercises at high speeds and calling it tech-something. Simplicity and complexity are difficult to handle in different ways, but neither is the whole of the end result.

Chances are the hardest thing for each person to compose well is what they label as the easiest to compose - if you don't understand the music, it is very hard to do it well yourself. Few people here understand and can compose hip-hop, j-pop, or trance, and listeners of those genres are likely to look at metal as shouting and noise - just because something is a type of metal does not mean that it can not be as foreign as any other type of music to you.