Fjord Cutter: It doesn't really hurt stuff, I think; it only chills (and occasionally freezes). The only use for this is, to my knowledge, on a support barb to keep up the Iceblink gem for a group; but there are better cube powers and better ways to get chill effects, so it's not really something you should cube.
Leoric's Crown: It simply takes the helmet gem and doubles its effect, no matter if you wear Leoric's Crown or have it in cube. Just forget about the orange text (which obviously can be misleading in that case).
Arcstone is useless. So is Shard of Hate. There's many much better rings+weapons.
Yes, Nemesis Bracers are very good, especially if you're using In-Geom they're basically a must-have.
Someone made a list of all useful cube powers a while ago, check this out (it's a bit older but should still be up-to-date):
If you're not sure about an item's usefulness, look in the build tool how many builds there are with said item, that should give you an idea:
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