#1 Too many world-class players to negotiate with for any opposition
Yes, Cristiano Ronaldo had a quiet outing in Kiev. But that's okay because there's Gareth freaking Bale. The Welshmen scored a Shaolin Socceresque worldie while being suspended 4 foot high in the air and trying to make contact with a ball that was falling a good couple of meters behind him.
Karim Benzema was linking up with his teammates up front and doing everything in his power to keep the pace of the game from falling. Marcelo's first touch and movement was helping Real Madrid find spaces that wouldn't otherwise exist.
Sergio Ramos was up to his usual tricks (read: dark arts) and Raphael Varane was a beast at the back allowing bare minimum to slip past him. Keylor Navas was solid in front of goal. When every player in this Real Madrid side decide to step up, there seems to be no team in Europe that can stop them.