I think that it's an understated strength of FFXIV that they can write such strong 'simple' villains, that are just focused on one thing and that's it. If all you need out of a character is to be the villain of a specific story, sometimes the best angle is to just make a character that can do that one role really well and that's it. Nidhogg, Vauthry, Yotsuyu, even Meteion to a degree, basically every single job and raid quest antagonist fits the bill. Sometimes you don't need depth from a character, you just need a really good villain that you hate; sure, they can't do much else, but maybe they don't need to.
To a degree, Zenos in Endwalker shows the struggles of trying to make a focused character like that long-term, though. He's actually better suited than most to getting further stories, because ultimately his focus was us (imagine trying to write for Nidhogg in any other expansion and you get what I mean), but that doesn't really give him that much more to actually do, so much as more places to do it in.