Well... when you have a Pokemon like Wobbuffet, what happens? Shadow Tag traps you. So then you're stuck. You could stat up to make your Pokemon stronger, use a stat-dropping move, a status problem move, or attack it directly. Either way, unless you're putting Wobb to sleep, chances are she will use Encore. Then what? You're locked into using the same move, and you're locked into battle. That means Wobbuffet could Safeguard, and then switch out to a counter of the Pokemon you have in battle. Either that, but if you chose to attack Wobbuffet, they'll know your next move and can Counter/Mirror Coat it.
Of course, there are ways around Wobbuffet. She's very slow, so putting her to sleep works temporarily. Taunt forces her to use Counter/Mirror Coat for a few turns as you set up your Pokemon. Spiritomb (and Sableye) are completely immune to both Counter and Mirror Coat. You could also use Trick/Switcheroo to swap a Choice Band/Choice Scarf/Choice Specs on her, and that would completely ruin her for that battle. So there are ways around her... it's just that she can be frustrating to battle.
And Garchomp is the other non-legendary uber Pokemon. Garchomp as great Attack, Speed, Hp, access to Swords Dance, a large physical movepool, etc. An Adamant Garchomp that uses Swords Dance and is holding a Life Orb is able to hit a lot of Pokemon for a lot of damage. Also, she has enough raw Attack to use the Choice Scarf, making her faster than a lot of foes and sweeping them quickly. Yache Berries are also common hold items for Garchomp as they allow it to survive an Ice attack.