(Spoilers up to ShB) So, what exactly DID our Urianger DO?

[Rosenstrauch]

I also think people give him far too much blame for doing it again in Shadowbringers. It was the Exarch who pulled him aside, told him a secret that he can see is far too important to break and is basically "well you've got to be part of this scheme now or a hundred years of planning and the fate of two worlds may be doomed, especially if you tell your friends the truth at this point. Thanks for agreeing to help."

I think part of it is that the Exarch's scheme had significant issues with it, such as his presumption that the WoL wouldn't accept his sacrifice and his failure to take into account Emet-Selch. Then there's the part where it backfired horribly. Without Emet-Selch enabling the finale and several miraculous events—Urianger happening to know a powerful Fae who can grant us passage to the bottom of the sea, Ardbert rejoining with the WoL, and the Lightwardens' Light being cancelled out when it struck Emet-Selch—there'd have been at best no saving the WoL, and at worst no saving the First. Neither Urianger nor the Exarch should be given credit for most of these things.

This also tends to get forgotten, but according to Ryne she was going to have to deal with the Lightwardens if the WoL's summoning failed. With that in mind, it's hard to believe that endangering the WoL's life by having them shoulder the entirety of the burden was even the best course of action.

But I think the real reason Urianger ends up getting blamed for all this is because he was the one left holding the bag, so to speak. That and he actually got down on his knees and apologized for it, whereas the Exarch never really had the chance to. For some people, the admission of guilt is a bigger sin than actually doing something wrong, as absurd as it sounds.