I'm planning out one of the most complex data migrations in WordPress I've ever been tasked to do.

I'll have to switch multiple custom post types to posts, map old taxonomies to new ones, AND map old ACF blocks to new ACF blocks. (Thank goodness this WordPress install is still using the classic editor and not Gutenberg, which would be a whole another layer of complexity.)

I've written before about migrating from Craft CMS to WordPress using a mixture of manual techniques and web scraping. (I have since improved this process so it doesn't overwork the computer. XD)

I find a shell script with WP CLI can solve a lot of tricky problems, too.

I'm super curious how other people handle these sort of complicated migrations? What are some things to watch out for? How did you test the results? Is there an automated way to test the results?