Warning! Spoilers for Batman/Superman #15 below!
Some missions are even too dangerous for Batman has his friend's best interests in mind, telling him he shouldn't take part in bringing the reincarnating villain back to his home directly, otherwise, he might risk death.
Solomon Grundy first appeared in All-American Comics #61 (1944) and was co-created by Alfred Bester and Paul Reinman. Originally from the 19th century, Grundy - who is loosely based on the classic nursery rhyme - is reanimated as a massive grey undead monster. Grundy has an impressive powerset that's included super-strength, practical invulnerability, a healing factor, and the ability to come back to life after death with even newer powers with varying degrees of intelligence. Grundy's existence has also been based around magic. The character has appeared in the live-action series Arrow, Gotham, and most recently in Stargirl.
Batman/Superman #15 by Joshua Williamson, Andrei Bressan, Alejandro Sanchez, and John J. Hill shows the titular heroes visit Arkham Asylum on one of the snowiest days of the year. Batman called the services of his Justice League ally in order to help him stop Solomon Grundy, a literal bomb filled with radiation. If he explodes, he could take out half of the United States. After consulting Poison Ivy, it's determined that the magical and science-based villain would be best returned to his home in Slaughter Swamp where he can regenerate. Superman offers to fly Grundy and be back "in a minute," but Batman recognizes how dangerous the grey monster's radiation can be for Superman, telling him "he's much too volatile, Superman, we can't risk your level of speed detonating him - and who knows what the blast could do to you."
Superman is arguably the strongest superhero in comics, but even he has weaknesses. While the Man of Steel is famously weak against kryptonite, his other big weakness is magic. Considering Grundy - at this moment - was a toxic mix of magic and science, there's no telling what an explosion with the radiation would do to Superman. Batman didn't want to see his friend potentially get killed when there were alternatives to transferring Grundy from Arkham back home. Ultimately, Superman and Batman help fend of the Secret Society of Super-Villains as Grundy is successfully transferred back home where he merges with the Green (with an assist from Swamp Thing) as the toxic energy is contained.
It's not a surprise Superman would risk his own life in an attempt to stop half of the United States from being obliterated by Grundy's toxic inner atomic bomb. However, Batman realized how vulnerable Kal-El was and came up with an alternative solution to getting Grundy back to the Slaughter Swamp. If Solomon Grundy is reincarnated with similar magical properties, he can be a massive threat against Superman - but hopefully, returning him to his home bought the Justice League some future goodwill from the villain.
