I think she's brutal, callous and power-hungry. This isn't to say that she can't be loyal to to people who impress her, but when exactly does she demonstrate empathy? I don't recall that directly.
If you talk to her at Skyhold about the mage rebellion she'll tell you that she empathizes with how they felt, but really could they have chosen a worse timing to do what they did?
A lot of what people hate about Viv is really just her realism - Cole provides insight into how she forcibly shuts herself off from becoming overly emotionally attached to many things because it's a weakness, both in the Circles and within the Orlesian court, and she's had to fight to survive (tooth and nail) - and she exemplifies the type of person who believes in changing oneself to adapt to the world you find yourself in rather than trying to change the world to adapt to you. I don't think there's anything wrong with that.
Does she want power? Yes - because she believes that she is better equipped than most to utilize power to influence society for the better. You see that a lot in her conversations with Cass - how she implies that Cass has a responsibility to take the power that Cass, as a hero and a symbol herself, possesses. Viv's flaw here is really her arrogance in believing that she can and will use power more effectively than most (but again, she also will defer to people she respects like the Inquizzy or Cass - there are just very few people she views as her intellectual equals/superiors)
She isn't callous - there's banter with Cole that implies she was worried about him (after Cole's personal), but is simply unwilling to express it. She also *mothers* IB if you have them in the same party (that one banter where she's scolding him for picking at his wounds is so adorable). Again I think you have to take her breadth of emotional expression within the context of her life - she's spent so much of it in a place where you have to live semi-permanently under a mask and emotions can be fatal that she's learned to discipline her own expression militantly. (This is really obvious after Bastien's death - it's obviously devastating for her, but she doesn't really let anyone express their sympathy and brushes all of it off with "it will be fine, dear" - which puts a lot of her reactions to the experiences of others in context)
Viv is arrogant and narrow-minded about approaches to magic/theology that are external to what she was taught and yes, a royal ****** in the best way possible, but she's never unnecessarily brutal (where have you seen her use unnecessary force relative to what seems par for the course at Orlais?), she's not an emotionlesstramplesonallyourfeelingsbecauseIcan pinnacle of callousness, and really there is nothing inherently wrong with wanting power/having ambition.
I honestly think a lot of the "Viv is a terrible human being" sentiment comes from players viewing themselves as revolutionaries and being jarred by having a companion who is a reformist but inherently counterrevolutionary. She's not the world's nicest person or most altruistic person and she can be a manipulative ****** and she's downright terrible to Blackwall (which turns out - deserved? idk Blackwall leaves me with complicated feelings), but in the balance she's still someone who cares about the world she lives in and is willing to put her life on the line to protect it (it's very slight but you can see hints of her unbending towards Cole - at one point Cole is all excited because she referred to him as "he" instead of "it" and Viv gets all denialmode). Maybe it's because I'm a type A economics major who has a serious bitchy streak and appreciation for a well cut silhouette and excellently thrown shade, but I can respect the kind of woman Viv is.