Zumataurus/bonelord/MTK drop a few good items, gems etc but it seems like that's all they drop. Still worth the trip for a solo player but not for a group.
And as far as I know there are only 6 high end bosses in the 1 drop file? That's orc warlord, evil emperor, jinchon devil, yimoogi and omakingspirit and great fox spirit.
The mobs in frozen hit very low, the floors are short and there aren't many of them, the boss can be killed by 2 people who play it right or four who just stick an archer or wizz in the corner so really it isn't on the list for a reason.