That's because they are not written in the changelog.
From 5 to 5.1 the way dates are summed changed. In the changelog there was a point in changing the duration of a year, but not changing how dates are summed. This triggered a lot of problems in a software I worked on involving calculations with time and money.
Finally, rails upgrades are always painful, unless your codebase is written by engineers that know how to avoid those pains. In which case, you probably will end up without rails (a lot of the features are useless to highly experienced engineers)