Really depends on your code base and project. Unreal upgrades can be painful if you're using APIs that can or are rewritten, and if you have made engine level changes.
Unity often feels like rolling the dice with upgrades, basic things can break from version to version, but I've done many Unity upgrades with no or minimal issues, and seen extremely painful Unreal upgrades
It’s true that you really, really want to avoid making changes to the engine. Even for AAA studio source licensees in prior UE generations this was widely agreed upon. The good news is, you really shouldn’t have to.
In my mind, the big win from having the Unreal source code is understandability. But also, in contrast to Unity, if you should for some reason need to—say a critical bug that’s stopping your game from shipping at the last hour—you at least have the option.
Every AAA source licensee that I worked at made large changes to the engine source. There are many AAA titles both shipped and in development that change fundamental things about how the engine works. Look at the GDC presentation about Mortal Kombat rollback network, or its predecessor about running UE3 at 60 fps on console for some good examples. Completely avoiding engine changed isn't the reality for AAA, and I imagine it's only true for small indies. Good news is it's pretty easy to merge small engine changes.
Unity often feels like rolling the dice with upgrades, basic things can break from version to version, but I've done many Unity upgrades with no or minimal issues, and seen extremely painful Unreal upgrades