"Crossing parallel universes requires a lot of energy; it has been calculated that each opening requires drawing enough arcane energies, channeled using the Crystal Palace itself as a giant mana-superconductor, to power an entire city when converted to electricity, and that's only to catalyze the process that would allow the portal draw even greater source of magic from the background magical field needed to create the link between dimensions.
Since we don't have city-powering arcane power planets, each transit would require a long recharge after each use, and have only enough to maintain a portal sufficiently stable to send a pony-sized object through and back each time.
It's not surprising, really, how difficult it is to use this portal; the quantum theory of parallel universes states that it is suppose to be impossible for information from one parallel universe to transit to another parallel universe. But no-pony told the portal maker that, did they?
Now, one can only imagine what the maker used it for, and what potential applications we can have for this if we can solve the power generation problem..."
--- Professor Magos Thaum, Ph.D Arcano-physics, currently leading researcher of the project investigating the Crystal Empire portal mirror.
I know. They did mention the Crystal Empire mirror was a prototype or something.
To be honest, the comic series, and the rest of the FIM expanded universe, had already wiped out a LOT of my headcanons (Though it is reasonably good in quality). I'm starting to consider ignoring it...