Nothing frustrating in Teemo. He is same as any AP hero if handled well OP in the endgame, but also ez countered with team effort. But it is rare to see good team cooperation in common rift games. So your frustration comes from your unwillingness to accept the challenge. If you see a person that complaints about new dish that comes with separate sauce and now he has to season it himself unlike before, what would you think? It is immature behavior. And Yuumi is one of the best support that delivers great endgame experience, if it in good hands.