Error handling was a huge issue, along with other weird distributed system bugs. Backed up queues, job shedding, thundering herds, you name it. When you have jobs on queues kicking off new jobs on different queues, tracing issues is just miserable. Sure, it's not a problem of ruby per se, but engineers would basically just throw their hands up and say "ruby can't handle this" and sidekiq became the One True Way™.
Maybe "ruby can't handle this" was a short form for "we can't run this in the Rails controller because the response would take too long" possibly calling 3rd party APIs, "and we would run out of threads."
Anything running in sidekiq is written in Ruby too.
