I had a similar nightmarish experience migrating a few Rails apps from 4.x to 5.x. Because of the complexity and size of our apps, it took a few months to successfully complete and test. Luckily we had copious rspec tests with decent code coverage. I too dislike the troubling trend of not being able to directly access JavaScrips directly from the view; I am not sure why this is done, but it makes debugging much more difficult.

More abstraction is not more better.

In any case, we will eventually have to migrate to 6.x, so that will be fun.