Here's an interesting two, actually: Lurantis and Toucannon.
Now for the first, if there's one ability I love, it's Contrary. Contrary makes Serperior an absolute monster with Leaf Storm and Malamar becomes a weird, fun attacker with Superpower.
So when I saw a Pokemon available in USUM that could actually USE Contrary and Leaf Storm in conjunction with one another, and it happened to be Lurantis, one of my favorite designs in Gen 7, I knew I had to use it in my playthrough. So I bred one and gave it all it needed to be an insane attacker through the game. He was too slow and he died to everything. I ditched him ASAP, because as much as I thought he'd be a useful partner, he just ended up being a punching bag. If he were just a bit faster, like Serperior, he'd work PERFECTLY. And in fact, speed leads us into Toucannon.
Now if there's another ability I love, it's skill link. It makes Cloyster my premiere pick for taking on the Battle Tree in gen 7 when I did. It's an ability that makes formerly useless moves legitimately useful. Toucannon is able to use moves such as Rock Blast and Bullet Seed to its advantage, so I figured he'd be fun to use, right?
He was too slow and died to everything. I ditched him ASAP and actually grew to hate his design somehow along with it. Sorry Toucannon, you tried.
Notice a trend here? These pokemon with HUGE potential for fun partners to use through the run are so incredibly slow and frail (ESPECIALLY lurantis) that they just cannot function properly. Their flaws are what made the time I used them absolutely miserable so I just had to ditch them.