Since I hate the tiefling, and don't want to level in rogue, I cross-trained in disarm device. But most traps in the game won't disarm even when I beat the DC. What gives?

I don't know if this is the case for you, but a certain level DC of trap can only be disarmed by a Rogue. It's pretty high though, so maybe that's not your problem.

This is happening with many traps, including those with low DCs. It is inconsistent, though. Some traps can be disabled if the DC is met or exceeded, others can't.

According to the manual, anything over DC 35 can only be disarmed by a Rogue.

It's happening frequently with DC's in the twenties, and sometimes with DC's in the high teens.

No idea then.

I must have remembered the high teens incorrectly. However, the readme file says:
p. 85: Only rogues can disarm traps with a DC of 20 or more.Oddly, I think I was sometimes able to disable devices with a DC over 20. Maybe it's applied inconsistently? Or perhaps I'm just remembering incorrectly again. Anyway, with take 20 rules, if you're not a rogue it's not worth taking more than one point in disable device.

Unless you want to try during combat :)

I never encountered anything I *needed* to unlock during combat, though perhaps there is something in the game like that that I have not encountered. Or, of course, if it fits your play style. But otherwise I think the points are better spent elsewhere.

Hm, I know the automatic take 20 outside of combat was removed, but you're saying your check result was higher than the DC and still you failed? Weird.

It's like in PnP...

If one of the knowledges isn't a class skill, you can only find information of DC10 or lower using it, even if you roll 50.

I think it's DC10 anyway. Might be 15.

Only Rogues get the Trapfinding feat(at lv1) that allows them to overcome traps with a DC of 20 or more. Any other class will, regardless of how many points are in the skill, fail.

As for your experience of failing while in the teens, well, thats off. Was it consistent? it could have been a bad roll or a critical fail?

Unlike NWN1, it also seems that you do not automatically take 20 when disarming a trap, even if not in combat.

That's the way it's supposed to be. NWN1 had it wrong. Take 20 only applies to Open Lock I think.