What I see as mixed attackers (or at least a successful ones) are mons like Iron Valiant, doubles Mega Salamence as you described, Mega Diancie, and Mega Garchomp (though I get the sense that Mega Chomp being a successful mixed attacker was unintentional on Game Freak's part). Pokemon that have it in their best interest to run mixed sets.I'd be interested in what your definition is for it, since in my opinion a "mixed attacker" is simply a offensive oriented pokemon that has close enough special and phisical stat (and possibly, the movepool to accomodate it) to be able to feasibly run both phisical and special moves at same time despite investing in only one (or even investing in both).
...Which 90% of the time ended up in still using only a single stat (see Dragapult or Greninja) because realistically speaking unless there's very specific movepool situations or you are looking for a lure set, even if you have exactly equal spatk and atk, you'd rather fully invest in one of them to be able to hit harder without nerfing your defenses or speed via nature.
The only similar case I can really think of recently was Mega Salamence who (mostly in doubles) would go special with a -def nature in order to have access to both a strong Hyper Voice / Draco Meteor, but also being able to hit hard on phisical side with Double Edge / Return or Earthquake, and even that was mostly due to the fact that you had no good special normal/flying stab, but also no good phisical dragon stab (unless you fancied locking yourself in Outrage or risking Dragon Rush misses), and in a scenario where Salamence had both reliable +spammable stabs in one direction, it'd really just rather have gone full phis or full special.
Which is basically Iron Valiant's scenario, where if it had Play Rough or a special high BP stab that didnt feature 30% miss chance, it's unlikely people would even consider going mixed outside of some lure sets
Keep in mind that I don't see "successful" and "viable" as meaning the same thing. Dragapult and Scovillain are very viable Pokemon, but they're not successful mixed attackers. Cacturne's viability may be dogshit, but I still consider it a successful mixed attacker.