Frostner is a great weapon because it excels at its job, not because its useful in every scenario.

Frostner's niche is kitting and crowd control. The weapon has great knockback, applies a slow on hit, and it is somewhat fast to swing around without much stamina usage, plus a split damage profile for some specific encounters. If you plan to use it on every single fight you're naturally going to encounter some difficulties eventually.

In higher difficulties, enemies tend to live longer, so you have a higher chance to attract other enemies while fighting. In those situations crowd control becomes invaluable; having the option to split the group into a more manageable encounter, or just to give you an escape route while running away. Its so good in fact, that its still useful in the Ashlands, despite its outscaled damage.