"You have to be taught how to fly a TARDIS," he said, following her quickly. His books were left forgotten on the floor where he'd dropped them, too busy chasing Elizabeth to worry about them. "And they're regulated! Well, they were, but the point is that a TARDIS has special methods and it doesn't make any alterations to the space-time continuum in such a way that it could damage more than one universe at a time." As the words fell out of his mouth, he realised exactly how bad they sounded.
Skidding to a halt near the table, he said, "The Daleks and Cybermen may not be in the Tears you open, but doing those... you put people at a very high level of risk. I've no doubt that maybe in your place, they're... they're different. Maybe there's something about the fabric of your reality that makes this," and here, he paused to wave his hand vaguely in front of both of them, "less dangerous, like my TARDIS is less dangerous in my world than it might be, say, here. But the point is, that could rupture and destroy not just your universe but trillions of others."
He realised he maybe wasn't being quite fair; the rules here were decidedly different from other universes. His sonic worked at minimised capacity, he had no TARDIS, the rules of physics may have changed drastically. And as he'd said, maybe what she could do was insulated in some way in her reality; maybe her reality was one that had lots and lots of pockets to stick its universes in.
"I'm sorry, Elizabeth," The Doctor said. "I know I'm not being quite fair to you. I don't know how your Tears work, and you don't know how my TARDIS works. I've just seen some really awful things come out of the impossible sorts of things you can do. And I don't want anyone else to be hurt by them. These are things that devastated my home, things that are unspeakably evil, things that want nothing more than to wipe out every living thing in the universe."
He put his hands down on the table. "I'm not trying to insinuate that you'd bring them through. I'm sure you don't have any plans to that nature." Reasonably sure, anyway. "But the fact remains that that is dangerous."