I'll concur that writing evil is harder. Moreover, other than Kotor, I don't remember any game that had any actual evil path. moreover, properly defining an evil path is hard. Even the drow city in bg2 and Korriban were not "evil paths" per se, but a mandatory parts of their respective games that had an option for evil flavour, but playing evil in them didn't change the story drastically.
With all that in mind, I think the fact is that Larian didn't have to tell the story in the way they did. It is much more difficult to create two storylines than creating one storyline with good/evil nuances. But if they did, I expect the same quality of writing from both.