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.
I am currently playing through BG 2 and took the vampire side just to be edgy and I'm playing chaotic neutral so thought it was appropriate. Surprisingly even Bodhi The Bitch Queen gives you a choice between a good and evil quest to further to her final quest.
Minthara should be designed similarly where instead of her being default evil with evil quests, we get the choice, whether outright or through deception, we can go along with the evil quest or redirect her to the "good" or "neutral" one since it is "good" according to an evil character.
The BG 2 quest lines required this flexibility to keep a party together for the PC in the early game and I think it is a good compromise for creating the branching storylines that are associated with alignment but also not requiring that someone just blindly go along with whatever they are told.
Honestly Larian is thinking too basically about their evil NPCs and need to get more writers who are enthusiastic about writing evil and neutral storylines.