I think the upgrade path from 2.3 being terrible is generally accepted as being true. But I don't remember any hard problems from 3.x onward.
At least for Rails itself. Gems dependencies are another problem.
Like at one project we had pains every time Rails upgraded their minor version because the previous devs thought using Squeel instead of the builtin ActiveRecord a good idea. Just for being able to write slightly "nicer" queries and now this is a major stopper for going to Rails 5.
Generally speaking at that job, though, we tended to not wean ourselves off the deprecated features - we'd use the extracted gem to keep the functionality going. Which works fine for one minor release, but doesn't work when you are 3 minor releases later.
Strong parameters is one that hurt bad. We used the workaround gem to avoid that for a long time, and it just made it more painful when we had to get rid of it.
I think generally there were a lot of related gems that were hard to get updated along the way from 3.2 to 4.0 as well. Seems like a fair amount of libraries were late to the 4.0 party, then jumped ahead to 4.1 or 4.2, and never really got ironed out well against 4.0, so you'd have goofy compatibility issues.
Squeel was a related problem that was horribly painful to remove from that stack as well, I forgot about that.
I think the upgrade path from 2.3 being terrible is generally accepted as being true. But I don't remember any hard problems from 3.x onward.
At least for Rails itself. Gems dependencies are another problem.
Like at one project we had pains every time Rails upgraded their minor version because the previous devs thought using Squeel instead of the builtin ActiveRecord a good idea. Just for being able to write slightly "nicer" queries and now this is a major stopper for going to Rails 5.