Ivermectin, the active ingredient in Heartgard, is the first practical compound approved for use as a heart worm preventative. It is widely used, not only in dogs and cats, but for livestock to treat many parasitic conditions. It's also approved for use in humans to treat river blindness.
I say that to point out that it's likely not the compound itself that is responsible for any possible ineffectiveness. If there is a real problem, my opinion is that it's probably in the administration by the owner or a manufacturing problem that may have affected some lots.
Once you get to the point of a lawsuit, it's hard to determine anything until a full airing of the case. All you really have are allegations and defensive responses to them. There may be real merit to some points of the case or not. I don't think it's true to say that Ivermectin is ineffective, however.
Here's some links on Ivermectin....
The History of Heartworm Medication | eHow.com Ivermectin ? History of Safety and Efficacy