The only time Fell Stinger has actually been used on a serious moveset (to my knowledge) is Mega Beedrill, due to its absurd Attack and Adaptability pushing the power even further, meaning it can pick up the +3 boost when the opponent still has a ton of health remaining. However, Mega Beedrill has almost always opted for other moves. In Generation 6 and 7 UU, Beedrill ran an all-out attacking and offensive pivot set, respectively, while in Generation 8 and 9 National Dex UU, it is wholly unviable. The reason for this is that Mega Bee already struggles with moveslots, wanting all of U-Turn, Poison Jab, Knock Off, Drill Run, and sometimes even Pursuit. If Fell Stinger is slotted in, then Beedrill will either have to sacrifice valuable coverage (remember that Bug/Poison is awful offensively) or U-Turn, a momentum tool that it values in Gen 7 UU.
As an overall move, Fell Stinger is not that viable. First of all, the opposing Pokemon needs to be at quite low health (unless it's Mega Beedrill using it, but we already discussed that.) Secondly, most Pokemon do not want to sacrifice a moveslot for a +3 Attack boost that might happen. With how abysmally weak Bug as an offensive type is, many Pokemon don't even bother trying. Sure, Buzzwole may seem like a great user of it, but Buzzwole would likely value other moves, such as Leech Life, Drain Punch, Earthquake, Ice Punch, or Roost over it. Finally, if you want a physical sweeping Pokemon, go with other fast threats with Swords Dance or, even better Dragon Dance Pokemon. These Pokemon are much more reliable, and can even abuse support from the likes of dual screens and Encore.
