Info from kripparrian's video
Let's say you're playing a sorceror or necromancer, so your main stat is intelligence, which also grants resistance to all elemental, poison and shadow damage. You probably have a bunch of paragon glyphs that grant resistances, and your gear grants you additional resistances too.
You can go to your build stats with advanced tooltips on and observe the stats of your individual elemental resistances. The game will show you that your current resistance stat reduces incoming damage of that type by x%.
If you were to now swap your gear, skills and points over to something different at random, and check this stat again, you will find that actually the majority of the damage reduction you had for the same damage type is still there, despite removing all your resistance buffs.
Most of the resistance to a given damage type comes from wearing any item at all, so relative to an item which prioritises the resistance type in question, the difference in damage reduction for that damage type is almost negligible. After investing all of your gear, glyphs, and skills into resistances, on the classes with the most resistances in the game, the difference will be damage reduction of a few %, for that damage type only.
Comparing to non-elemental situational damage reduction stats like 'damage reduction from close', kripparian's calculations showed that a perfect roll on an elemental resistance item still yielded significantly less damage resistance to an elemental damage type than a typical 'damage reduction from close' roll yields for all damage types (it is situational, but most enemies in the game are close).
Given there are 6 forms of elemental damage in the game, for the specific items compared the resistance item yielded less damage resistance overall by a factor of 33x. For most of the other stats in the game, they are in a roughly similar ballpark in terms of efficacy, except this one which is over an order of magnitude worse than really anything else.
It's a wasted stat. If you tiered out all the possible stat rolls for all of the items in the game for any class, for any item where an elemental resistance is possible it is the worst roll you can get. The class worst affected by this is necromancer, followed by sorceror, and kripparrian goes on to point out that these are currently considered the worst performing classes in endgame
Just a PSA really, I think a lot of players are currently running a build that could be objectively improved by rerolling a resistance stat or replacing that item.