It gets dramatically harder when two balls appear simultaneously (especially if they are both the same color!)
Yup, it would be nice if maybe single balls just came with a bit shorter interval after a while, before jumping to two balls at the same time, or maybe that one can press space to get turbo time (sort of like "down" in tetris). First time I got to two balls they were same color, and I was so surprised that I didn't know what to do and missed. Second time I got same color in the second pair and even though I was less surprised, I still managed to goof up. Then you sort of have to "sit through" the first single balls just to give it another shot.
Otherwise pretty good gameplay and looks quite nice, but as Brackeen said, AA would make it look even nicer.