Desync isn´t always caused by the host. Sometimes the error comes from within and is caused by different mods.
E.g.:
Player 1 uses JSRS Soundmod.
All other players don´t use it.
Now what happens is that the client from player 1 tried so sync each sound that is generated by the game with the clients of all other players via the server. The server will check all other clients and as player 1 is the only one with JSRS, it returns a "sorry i don´t have that sound available" to the client of player 1. This happens with every sound, every second. During the same time the other clients are trying to communicate with the server as intended. Over time it causes a massive lag/desync.
This happens with other mods too but sound mods are the biggest reason for this kind of problems.
Try to check what additional mods you, your players and the server has installed. Ensure that only mods are used which are installed on the server-side.
If you want more information, google is your friend. Hope this helps and hopefully you understand what i was trying to explain.
/edit : If you have blocked "everything" via "BE" / "insert anti-hack of choice", then you can ignore the comment above.