Poppy I know fecals can't test for encysted red worm or for tape worm hence my horses get wormed in spring and autumn to which is the best time to kill them. I also religiously poo pick my paddocks and cross graze to minimise the chance of worms in my paddock
We do not have pancur powerpac over here however it is the chemical that is the active in panacur that the worms are resistant to, so you could pump your horse full of them but it wont kill the worms.
We have a limited number of drugs that kill worms, constant over worming is leading worms to become immune to them. What do we do when they are resistant to all of them?