SQLite3 can be used directly and is most convenient for server owners. The construction of SQLite3 is kind of like Config. However, SQLite3, when used in memory to manage in-memory objects instead of saving data (new \SQLite3(":memory:")), it has no alternatives. MySQL requires the server owner to setup a MySQL server and enter the host, username, password, port and schema name into the config. However, if it is connected with localhost, it has almost latency caused by network, and its data management is run at another process so PocketMine has less things to run. If you are making a plugin, you should let the server owner choose between the two.
I actually use MySQL3 for all my plugins, because it is easier to use and a lot less work than MySQL.
In fact, SQLite3 and MySQL are similar, but you have to think about how to minimize the number of requests in MySQL because it will get very slow if you don't.
Mysql is better because if you have multiple servers you can sync simpleauth or a stats plugin to all those servers to one database
MySQL calls is only great if both PocketMine and MySQL Server is hosted in one machine otherwise its crap.
If you use MySQL wisely, it isn't. You think it's crap most likely because you don't even try to minimize the connection.
Then try not to call. Try to use PHP memory as the database and the MySQL server only as an image of your PHP memory.
And when will you start working on StatsCore? I don't want to be the only one contributing in a team project.