Let's say for the sake of discussion that your MySQL db has a lot of tables encoded as "utf8." Are there any known drawbacks or gotchas to converting them en masse to "utf8mb4"? Is this a lengthy operation?
Schema changes to large tables are always huge pain in the ass in MySQL. Every team I’ve worked on goes to great lengths to avoid changing the schema, which leads to all kinds of exciting anti-patterns like recycling old fields even if their name no longer matches what is stored in them.
So you're right but also wrong thanks to good tooling. If you have enough discipline to not put triggers on your tables then pt-online-schema-change has got you sorted. Does it take forever? Yep, but does it matter that it takes forver? Nope because it doesn't lock tables.
And even if you do have triggers https://github.com/github/gh-ost you can still do it.