The thing there is that 'controversial' can be taken in two fundamentally different ways with characters like Venat and Emet.
1. 'This character stands for and acted on things that aren't inherently agreeable and cause split emotional responses as people have different reactions to them. This is both true and intentional for Emet and Venat, and are a huge partof their joint popularity; neither would be nearly as popular if Emet were just a sassy boy with depression, and Venat was just a nice wine mom. Even Harchefant's popularity is falling off over time.
2. That a character actually has a split response among the fanbase in terms of 'like' or 'don't like', to the point of significant divides. FFXIV doesn't actually have many of these on a large scale, and the best I can think of (for VERY different reasons) are Lyse and Hildibrand.
I'd prefer to use the term 'polarizing', because neither is actually, by dictionary definition, 'controversial'; none of those characters caused controversies.
These are entirely separate definitions, and one doesn't inherently cause the other. Again, Emet and Venat are two of the most popular characters in the game, in very large part because of their emotionally and morally complex stories and deeds. (Granted, this isn't exactly a perfectly repeatable formula; Ysayle didn't exactly see the same bump, and I have some largely unrelated thoughts on why.) And on the other side of that coin, the reasons Lyse and Hildibrand get hate is in no way related to their views and actions, that range from ineffectual to nonexistent. I think using terms like 'controversial' or 'polarizing' relating to Venat (doesn't really happen around Emet) is basically an attempt to conflate the two as the same, when they absolutely aren't.
You're allowed to not be okay with what Venat did, just as I'm not okay with what Emet did, and we don't have invalid opinions in thinking such. But this doesn't make these characters a failure, and it's wrong to approach them as if they did; it makes them successful. Nor does it make them unpopular; the thing I saw in that Twitter and Reddit dive, far more than actual dislike, were people talking about that part of Venat as a big reason why they like her.