@OP I had a similar incident with an anaconda. I launched all my pack hounds and hardly dented his hull.
Then I noticed I could barely keep up with it in my FDL lol. It was drifting along at 400+m/s with all its external modules destroyed lol.
I tried to ram it to death but the speed differential wasn't high enough lol.
Missiles are fantastic for taking out weapons at the moment. Target your opponents weapons with a seeker and a few missiles later they will have no weapons to shoot you with. I find that does tend to make them (NPC) high wake though so be ready if you want the kill.
Yes, I noticed that targets moving away at high speeds tend to be a lot more difficult (impossible almost) to damage. There are some oddities that you need to get used to with pack-hounds.
I did some testing vs thrusters, point defence + ecm, power plant and misc ships.
[video=youtube;R0IOCp_AAXI]https://www.youtube.com/watch?v=R0IOCp_AAXI[/video]
If you just want a summary, they work fairly well for taking out thrusters, external modules and smaller weaker ships.
Bigger ships, and ships moving away from you at higher speeds are more difficult to damage. Targeting the power plant seemed pointless.