Rend doesn't ignore armor, it ignores defense. What it means in practice is that Rend hits through any kind of blocking, be it a shield, sword, two handed weapon, crouching golem etc. To top it all off, Rend is the only way you can ignore enemies attempts to block your strikes. This is why I consider Rend to be the very best skill in the game. Very very useful, a must if you ask me.
Sunder armor... Yes, it lowers the damage resistance, but as with any kind of armor penetration its usefulness has a very wide curve depending on how much armor your opponent has. Sadly we don't have that many heavily armored baddies in this game. Most notable are Elementals/Golems, Wild Hunt Warriors and insectoids with their chitin. So the skill is also situational, to make the matters worse it doesn't last the whole fight, in fact its effect is quite short, can't say how long exactly. So I say the best way to deal with heavily-armored foes is not this skill but anything from this list:
- Going for as high DPS as possible.
- Getting gear and/or runes with inherit armor penetration.
- Archgriffin Decoction - it targets percentages rather than absolute numbers.
- Forgetting about it all the way, they'll fall anyway