Pantheon. Holy heck, don't even get me started, but its 'release' on AMC was a crime against humanity.

...I've always thought Avatar was on the wrong network. ATLA was literally about ethnic genocide, but it never really got to confront the horror bluntly or get messy (which I guess the adaptation is trying to get at). LoK spun some dark, complex threads on politics and PTSD, but there just wasn't enough screen time to tug on them between 'kid stuff' filler.

I wouldn't call them failures though. They're two of the best series ever.