I've done major upgrades from 3.2 to 4.1/4.2, and from there to 5.1 followed by 5.2 - the only time we had to put in a little work was when moving from 3 to 4.

I've done major upgrades from 3.2 to 4.1/4.2, and from there to 5.1 followed by 5.2 - the only time we had to put in a little work was when moving from 3 to 4. When moving from 4 to 5, we relied on simple smoke tests and unit tests, and had no major issues or bugs. The biggest effort was to make sure all of the application and environment configurations were up to date and using all the new settings introduced etc. My very subjective opinion is that either most of these code bases are low on quality (meaning they are harder to maintain in general), too tightly coupled with Rails itself (models stuffed full of logic, instead of using plain ruby objects for logic and keeping ActiveRecord for persistence level logic), or engineers are just too scared to make changes to the codebase - which again is perhaps a combination of bad test coverage and bad code quality. Either way the stories of upgrading major versions being a huge undertaking always make me scratch my head and wonder what are we doing wrong if its easy for us. And inb4 someone claims our apps are just small and simple - we run about 12 Rails applications in production in various sizes, about half of them being relatively large.