A small correction, before I answer question. The assumption
Liquibase seems to have everything Flyway has
isn’t correct. Flyway shines when it comes to parsing SQL. You can use unmodified SQL files generated by your native tools containing all kinds of complexity like PL/SQL packages and procedures, MySQL delimiter changes, T-SQL, PostgreSQL procedures, … With Liquibase you would have to split these in individual statements, add extra comments to the SQL files, …
The beauty of being able to use your SQL files as-is is that you avoid lock-in. You can take your existing SQL files, start using Flyway with minimal investment and moved away later if it doesn’t suit your needs anymore. Not so with Liquibase.
Also the issue of down migrations (think of them as compensating transactions, not rollbacks) is really something that sounds great in theory, but that is almost never needed in practice. See this old documentation page¹.
However when it comes down to using one or both, I certainly agree with SteveDonie (Liquibase team member) that using just one instead of the both together is almost always the better choice.
Disclaimer: I am Flyway’s creator
¹ Though Flyway does support undo migrations nowadays, by reading the old documentation you’ll understand the point Axel Fontaine is trying to make.