Since Sun Microsystems buy MySQL (then Oracle Corporation buy Sun), for MySQL it was time of big changes. It was a time when great changes was made and MySQL has grown and had power to handle many projects with small or middle size amount of data. MySQL is a great tool witch survive on market many years.
In my opinion something goes wrong. MySQL 5.7 was the last version which supports Query Cache.
The query cache is deprecated as of MySQL 5.7.20, and was removed in MySQL 8.0.
https://dev.mysql.com/doc/refman/8.0/en/server-system-variables.html#sysvar_query_cache_size
MySQL 8.0: Retiring Support for the Query Cache
Limitations of the Query Cache
I can understand changes, when the change goes to something better – but we don’t have nothing. None similar functionality.
Downgrading is not supported.
https://dev.mysql.com/doc/refman/8.0/en/downgrading.html
If You are lucky you can have problem with new collation for example:
When restorinng 8.0 db in mysql 5.7 – Unknown collation: ‘utf8mb4_0900_ai_ci’
What else?