BOAC,
As I understand it, you are using DDNS because your IP address may change, being dynamically assigned by your ISP.
DDNS allows your router to update the DDNS service if your IP address changes, so that the DNS name resolution always points to the correct IP address.
Regarding general security - well, you are no less secure than if you had a fixed IP address and used a static DNS.
You presumably have port-forwarding enabled for at least HTTP and FTP so you need to ensure that the server(s) accessible in this way are properly hardened.
If your router / firewall has the ability to create a DMZ for this purpose (hosting publicly-accessible servers), that would be ideal - thus separating the inside network from the publicly-accessible server(s).
SD