Generally, a table based sequence strategy is the most efficient strategy you can use on MySQL which lacks support for native sequences. The performance advantage of this approach is visible when you use a proper allocation size for the sequence i.e. 50+ because then, Hibernate won’t have to send every insert immediately to obtain the primary key, and instead can cache a certain amount of id values which it can use without DB communication. Now, when you insert many records at once, and Hibernate can transparently re-order inserts if you configure it to do so, Hibernate will use JDBC batching if enabled. JDBC batching is a very efficient way to insert many records at once, but that only works when the primary key is not generated during an insert by the database.
So overall, if you don’t need high throughput insert performance, you can surely use a generated identity, but by default, Hibernate 5 chose to use make use of the generally more efficient and more portable approach which also enables other optimizations. Not sure if you can switch back to the old behavior somehow, but this should really be a conscious decision you make, so I’d recommend you to make this explicit everywhere.