I've heard the comparison between Sombra and Sauron before but I think Sombra was somewhat.. the only word I can think of is lacking. I loved the episodes in all but Sombra needed to be a little more ominous I think. I don't think he's the most terrible villain ever, but I don't think he's the best ever either. I do get what you're saying and I agree each villain has their own defining trait, hell Megan Mcarthy even said she was inspired by Sauron when writing Sombra didn't she? I just don't know if they pulled it off completely with Sombra.
That being said I don't think they pulled it off with Nightmare either. Or even Chrysalis the whole time.
I can be pretty hard on the writers though.