Sometimes if you turn off fly hack close to surface (1-2 blocks) you can take a lot of fall damage and die (even with No Fall enabled). Workaround: before turning off fly hack, fly upwards (even 0.5 seconds can be enough). If you move upwards server resets internal fall distance value and sets it to zero. If you move downwards for a long time server accumulates fall distance, and that's why you take fall damage.

Enabled

Override max speed

Flying speed (default 0.05):

OnGround flag OnGround = false OnGround = true

While flying this module overrides onGround flag in MovePlayerPacket. Both options works fine in vanilla. For Fast Break hack, Disable Fly Penalty option you need to set OnGround = true.