Or Zacian-C in general. Good bulk, very high Speed and a seriously high Attack complemented further by Intrepid Sword. If anyone hates dealing with Mega Mawile, then that anyone would despise fighting Zacian-C. You would thought it would be slower with the heavy armor/blade and not going somehow faster?
Too keep it relevant to the thread's premise, here's a few Pokémon that disappointed me in-game when I used them.
Sableye (RSE): Back when I was a youngster, I am intrigued by Sableye's design and give it a try in a Battle Frontier thing (don't remembered exactly where). The result ain't pretty, and for a good reason. As I grow up, I became stunned in anger with Sableye's weak stats, and cannot evolve despite being available early. While it gained Prankster in Gen 5, it wasn't until ORAS that Sableye become in a more favorable position... and only in games where Mega Evolution is possible.
Castform (RSE): I don't exactly remembered when I used it, but I know that I somehow tried it before, with bitter results. Again, when I discovered it's low stats and not being boosted when in any weather, I was disappointed en masse.
To save everyone from disappointment in advance, don't use the following in RSE: Plusle, Minun, Sableye, Mawile, Volbeat (unless you want to mess with Tail Glow), Illumise, Castform, Kecleon, Spinda, Seviper (included it does to poor bulk yet slow speed) and especially Luvdisc aren't something you really want to use in long term because of poor stats and insufficiently good Abilities. Some like Mawile and Sableye got much needed improvement in ORAS, but others aren't so lucky.