WARNING: The following contains spoilers for Justice League #67, on sale now from DC Comics.
Arkham Asylum may be one of the notorious prisons in the DC Universe -- until its recent "A-Day" destruction and towering replacement -- but there's one DC Universe prison that's far worse than Gotham's. The Phantom Zone, the harrowing prison created by Superman's father, is miles worse than anything Batman's villains have to go through. This is something that the Man of Steel just had to remind the Dark Knight during a particularly hard-fought battle.
The devastating fight between the Synmar Utopica and the combined forces of the Justice League and the United Order comes to a close in Justice League #67 by Brian Michael Bendis, Phil Hester, Eric Gapstur, Hi-Fi and Josh Reed. With the powerful foe proving too much for these mighty heroes, Batman goes against Superman's wishes to secure victory. Taking the Phantom Zone projector from the vaults of the Hall of Justice, the Dark Knight uses it on the Synmar Utopica, trapping him in the horrific prison, seemingly for good.
As the heroes stand in the ruins of the Hall of Justice, Black Adam asks if this fearsome foe is gone for good. That is why Batman sent him to the Phantom Zone after all. Unlike Arkham Asylum, whose inmates seemingly escape with ease, the Phantom Zone has a reputation for being inescapable. However, Superman doesn't believe the prison is quite so secure. If the Synmar Utopica was powerful enough to escape from the custody of the United Planets and face the combined might of the Justice League, then he could feasibly free himself from his current imprisonment too.
The Phantom Zone can be a very effective prison sentence for powerful foes. Those who are trapped there don't age, don't need food or water and don't possess their powers there either, as they're essentially rendered incorporeal phantoms. They can see the world outside but have no way of interacting with it unless released by someone on the outside. At least, that's how it's meant to be.
Despite the reputation it's built up over the years for being a horrendous inescapable prison sentence, the Phantom Zone has a long list of individuals who have managed to break out from the inside. Its most famous resident, General Zod, has broken out on a number of occasions. Though some of these were due to outside interference, he was able to devise an escape from inside the Zone in Superman: Last Son by Geoff Johns, Richard Donner, Adam Kubert, Adam Kubert, Dave Stewart and Rob Leigh. This was by using his son Lor-Zod, who he propelled out of the Zone to Earth via a rocket. Superman himself escaped the Zone in a similar way in the same story, using an escape pod from Fort Rozz.
One of Superman's concerns is that the Snymar Utopica could escape because of his immense power level alone. While powerful individuals like Superman and Zod only escaped using tech, that wasn't the case for two of the most powerful individuals in the DC Universe. Superboy-Prime was briefly trapped in the Zone during Infinite Crisis. Unfortunately for the heroes of the DCU, he was so strong that he was able to simply punch his way out. When Doomsday was trapped in the Phantom Zone in the New 52, his body adapted so that he could tear his way out of the Phantom Zone.
That isn't even mentioning coincidental escapes like Supergirl and Batgirl getting some help from a friend inside or Wonder Woman recently entering and leaving by following Janus's Godscraper portals. Superman fears that this latest villain could punch his way out like Superboy-Prime or Doomsday. However, there are also easier ways out that are tech-based or even by chance. Despite its extradimensional nature, the Phantom Zone is arguably crueler than Arkham Asylum, and it doesn't do that much better at keeping its inhabitants in place.