Well, the most difficult project I had to upgrade was a 2.3 project with a custom Asset Pipeline, with some additional features that Sprockets do not support.

Well, the most difficult project I had to upgrade was a 2.3 project with a custom Asset Pipeline, with some additional features that Sprockets do not support. Sometimes the upgrading problems arise due to gem dependencies, as a lot of gems set hard constraints on other gems' versions so it can be a little tricky to update everything at once. All the other upgrades where rather easy, but of course it depends heavily on the size of the project, the test coverage or the underlying server infrastructure.