Solar nightfalls gjallarhorn destroys bosses way faster than black hammer, unless if it was maybe the nexus or a devil walker. For atheon yes black hammer does great damage, and even better with weapons but still you've never seen a team take down atheon with black hammers in 17 seconds have you. It only takes one missed crit shot with black hammer and it loses most of it's usefulness. That's not an issue with gjallarhorn seeing as you can't miss.