Personally, I think Sombra gets a bad rap, I suspect mostly due to what people imagined him to be in the long wait between when he was first hinted at and when Season 3 finally premiered. He was never meant to be chatty or catty like Discord or Chrysalis. From his backstory to his smoky present-day showing, it seems pretty clear to me that they were going for a ghost-king kind of character such as the Ringwraiths from "the Lord of the Rings" or Lord Soth from the old "Dragonlance" books. Hell Sauron himself served as the big bad through something like 12 hours of LotR extended films and was nothing but a hissing eyeball that said stuff like "I seeeeee youuuuuuuu" and then crumbled when the ring he wanted was thrown into a volcano miles away. I feel like Sombra did alright for a not-fully-ressurected-until-the-last-second smoke monster ghost in only 44 minutes of pony episode.
Still, I get that not everything is going to connect universally, and for a lot of fans he just wasn't their cup of tea. Still, I do have to wonder if he (or his horn) is ever going to make an appearance again. As it stands right now, he seems to be the only show villain who was actually killed off in his defeat.