Having looked at the guides they say things like "Yes this is a viable spec, however it is not viable on molten due to a bug with Deep Freeze (it does not do damage on bosses like it is supposed to, tested as of December 23 2013)."
Have looked through the Bugtracker and Changelog back to around 2015 and saw no mention of it. Has this been tested recently? Arcane and Fire are fine but Frost is honestly my favorite spec for Mage, in both sides of the game PvE and PvP..
Gear wise Frost should be almost, if not exactly the same as Arcane.
Rotation is more or less the same, aka spamming 1 1 1 1 2 1 1 1 1 2, so i dont see a reason why would u want to go Frost, since the dps diference is heaven and earth.
Also, i doubt u can outdps anything as Frost, even undergeared Warlocks and Shamans. :D
The only reason i said anything about it is cos i tryed it out, 1-2 months ago, and its simply no good.
P.S. Deep Freeze wont DMG the HC target dummy, but it will always DMG bosses, thats why some might think DMG part of the spell is not working properly.