I've never had problems with upgrades, in fact the latest version of Rails make upgrading a breeze. What Rails app version are you trying to upgrade? Happy to help. Rails also has an upgrade guide that's quite good: https://guides.rubyonrails.org/upgrading_ruby_on_rails.html
And you can use https://railsdiff.org/ to check what you're missing in upgrade apps. Anything other thing that breaks would most like come from another gem, which doesn't make it Rails' fault or Ruby itself, in the case of Ruby, you can track everything to the source to fix it. Ruby keeps a change log diligently. Everything is trackable, at least in my experience.
And you can use https://railsdiff.org/ to check what you're missing in upgrade apps. Anything other thing that breaks would most like come from another gem, which doesn't make it Rails' fault or Ruby itself, in the case of Ruby, you can track everything to the source to fix it. Ruby keeps a change log diligently. Everything is trackable, at least in my experience.