Reading through the books once again, I came to ask myself:
What "truth" is told whilst being affected by veritaserum? Is it the objective truth or what one believes to be true?
Regardless of how the answer is I see many problems arising from this, but also some great ideas.
According to JKR's own description of Veritaserum, use of the serum...
"...would have you spilling your innermost secrets". - Harry Potter and the Goblet of Fire
Although it may force you to accept a truth you've been denying yourself (that you were in love with someone, for example) it doesn't seem to grant you any insight into things that you don't already know. In that sense, it can only reveal the subjective truth, albeit one unvarnished by personal prejudices.
Note also, that a powerful wizard can hide things even from themselves so it's actually pretty useless as an interrogation tool unless the victim is caught unawares:
Q. Veritaserum plays a big part in finding out the truth from Mad-Eye Moody in book four. Why then is it not used for example in the trials mentioned in the same book? It would be much easier in solving problems like whether Sirius Black was guilty or not?
JKR : Veritaserum works best upon the unsuspecting, the vulnerable and those insufficiently skilled (in one way or another) to protect themselves against it. Barty Crouch had been attacked before the potion was given to him and was still very groggy, otherwise he could have employed a range of measures against the Potion - he might have sealed his own throat and faked a declaration of innocence, transformed the Potion into something else before it touched his lips, or employed Occlumency against its effects. In other words, just like every other kind of magic within the books, Veritaserum is not infallible. As some wizards can prevent themselves being affected, and others cannot, it is an unfair and unreliable tool to use at a trial.
Sirius might have volunteered to take the potion had he been given the chance, but he was never offered it. Mr. Crouch senior, power mad and increasingly unjust in the way he was treating suspects, threw him into Azkaban on the (admittedly rather convincing) testimony of many eyewitnesses. The sad fact is that even if Sirius had told the truth under the influence of the Potion, Mr. Crouch could still have insisted that he was using trickery to render himself immune to it.
Veritaserum certainly only makes someone say what they believe to be true, rather than what is actually true. It is easy to see that this must be the case: were it not so, someone at the Ministry could simply self-administer Veritaserum and find out where Voldemort was. Or Dumbledore could drink some and determine whether Voldemort had made Horcruxes. And so forth.
Further, we have direct evidence from the books themselves. While under the influence of Veritaserum, Barty Crouch Jr. says the following.
“I offered to carry the Triwizard Cup into the maze before dinner,” whispered Barty Crouch. “Turned it into a Portkey. My master’s plan worked. He is returned to power and I will be honored by him beyond the dreams of wizards.”
—Harry Potter and the Goblet of Fire
Of course, Barty Crouch Jr. ended up having his soul sucked out by a Dementor, not honored by Voldemort in any respect, so he could not have been speaking the objective truth.
