Writing evil companions is probably a similar skill as writing an evil storyline. At the very least, evil storylines contain evil NPCs, which means you have to write evil characters for both. In each, you have to balance 'evilness' while still making the companion/storyline enjoyable. The problem with Larian's evil path is that there is little reason to join Minthara besides liking murder and sex. One big worry Larian probably had is that it'd be too off-putting and no one would want to play it.
A possible problem with evil companions is that they'd be too over-the-top cartoonishly evil, where their entire goals were murder. Or act like jerks and no one would want them in their party.
