Admittedly, the giant units (mountain, frost and fire) are some of the most powerful units in the game. However, as with all powerful units, they suffer from having a high population count, making their unmitigated use somewhat limited. In addition, giants require a fair amount of favor to train (even for Loki: 27 - 32), a resource that doesn't always come cheaply for the Norse.
Nidhogg is a powerful aerial siege unit, and can turn the tide of battle for the Norse when fighting against massed ground units. Against archers and upgraded towers/fortresses, though, Nidhogg is quite vulnerable. Most players tend to 'back up' Nidhogg, using him strategically--which means micro-managing his movement-- the effort of which can either work for you, or against you. The Norse, with there general vulnerability to pierce damage, makes baby-sitting Nidhogg more of a bother than a help at times. Of course, a lot depends upon what the Norse player is up against.
I dont' think Hel is overpowered. Compared to the powers/units available only to Thor and/or Odin--Frost, Ragnarok, Valkyries, human regeneration, etc.--Hel simply makes Loki a more viable god in the 'late' game by playing to his strengths.
RW
[This message has been edited by Razorwing (edited 11-17-2002 @ 01:40 PM).]