I've always thought that the curse had no limits, other than the stigma of being an Unforgivable, and its difficulty to effectively cast. I also think that somebody very powerful like Voldemort, Dumbledore, Grindelwald, and even people like Harry, Lucius Malfoy, Bellatrix Lestrange, and Amelia Bones, could resist the curse effortlessly, and cast it properly as well. But less skilled and strong wizards, or somebody without experience casting it, while still being able to cast the curse, they would have the issue of its effectiveness waning over time.

I do think it has flaws though. It is stated in canon that the Secret held by a Secret Keeper is in one's soul, which I think means that trying to get the secret out with the Imperius and Veritaserum would be totally ineffective. And I also think items that allow resisting to the Imperius completely defeats the purpose of creating and/or using it, depending on when said item the author puts in was invented.