What's the disadvantage of encrypt mariadb?
Your question is a bit open-ended, but here are few things that might be considered disadvantages:


Encryption and decryption are CPU-intensive tasks, so you will see increased CPU utilization. If your server is CPU-bound, this may be a problem.
As a consequence of the above, encryption/decryption add certain latency to your I/O operations, so query performance might be negatively affected.
There is certain effort involved in managing encryption keys. Also, if you ever lose them, you lose all your data as well.