Just like what Emiliavance said, it's not a good idea to summon everything you have in your hand. This is true but even if the opponent manages to clear your monsters, they'll return to the extra and next turn you'll be able to summon them once again. This is what makes Pendulum broken, imo. Not the special summoning from the hand, but from the Extra Deck.
However, this mechanic isn't
strong if the deck itself doesn't contain many pendulums. Qli is strong because EVERY monster the deck contains is Pendulum. This means that you can be able to set up your scales a lot more often than usual. Think about this: why aren't DDD and Yosenju meta decks in OCG? Because they aren't pendulum-based decks. They usually work better without
their themed pendulums.
Another powerful aspect of Pendulums is how
the opponent should face the pendulum user. Years ago powerful cards got released: Fiendish Chain and Effect Veiler. They completely changed how monster effects could be countered, I think Divine Wrath was the only monster-negation card. You can't negate the effects of Pendulum monsters while they are in the Pendulum Zone; this shouldn't be relevant because they act like continous spells, so you just can use MST on the pendulum and "negate" the effect, in the same way you can "negate" cards like Tenki. The problem is that when you use your MST to negate the pendulum in the pendulum zone, the opponent's backrow remain untouched. With powerful Trap cards like Vanity's Emptiness, Skill Drain, Bottomless, Solemn Warning, ecc. , this is an HUGE advantage. And I think that maining cards like Spell Shattering Arrow and Fairy Wind just for pendulums completely slow you down in the same fashion MST slows you down when facing Lightsworns.
I know we aren't talking only of Qli, but what if Konami releases Pendulums like Helix and Carrier, which effects activate in the Extra Deck? How can you negate them? With Divine Wrath. And everyone knows why it's a bad card. Just think about the "when this card leaves the field"cards, such as Quasar, Absolute Zero and Reborn Tengu. Their effects, not counting the 101 and missing timing rules, activate whenever the opponent manages to get rid of them, but the strongest fact is that you just cannot negate their effects with staples like Breakthrough Skill, Effect Veiler or Fiendish Chain. The same happens with Carrier and Helix. I hope similiar effects won't come in Yugioh.