This has not been my experience.

I have taken applications from 2-to-3, 3-to-4, and so on, through 8.X.

If anything it has gotten better/easier over time. The most challenging upgrades were 2.X to 3.X (for reasons I can't recall), and then 6.X to 7.X (for an application that had issues adopting zeitwerk). In both of those situations, there was a lot of rote legwork, but once tests were passing, the application was working reliably. The other upgrades (3-to-4, 4-to-5, 5-to-6, 7-to-8) weren't happy-fun-rainbows-and-unicorns, but they weren't catastrophically complicated? Not even in fairly large codebases.

For each of these, a strong test suite was the best tool, which it sounds like you already know?

In my experience, the other impactful factors were:

(1) Reading the CHANGELOGs and knowing enough about Rails to know what they meant for the application.

(2) Using test fixtures (or fixtures with only some factories) for a quicker feedback loop for engineers.

(3) Having a true QA function in the company that isn't just engineers testing their own code.

> 2.X to 3.X (for reasons I can't recall)

Many big structural changes, IIRC bundler integration, plugins became engines, etc...

It was the big Rails+MERB merge.