[antsar]

The hard part isn’t updating. It’s checking what got broken by the update, and fixing that. Then finding out one of the plugins is no longer supported, researching alternatives, and rebuilding half the site to use a new thing.

[piotrkubisa]

Tip: static analysis tools for PHP helped me during a rewrite from 5.6 to 7.0. It was really good experience, I'd say.

[antsar]

Same here. Sadly, I don't know of any static analysis tools that can detect inter-compatibility of different versions of WordPress plugins/themes/core. That's one aspect of WordPress maintenance that can get real hairy.