It has to do with Herodotus not really being critical of what he's told, and including most things he simply deems worthy, whether it's for entertainment value or because he deems it important is hard to say. Thucydides, being one of his bigger critics, claimed that Herodotus didn't mind making things up for the entertainment value, and was just a story-teller - not a historian. He also didn't list a lot of sources, he mostly just mentioned who told him what he's saying, which I guess... is kind of a source? But not really? It's hard to tell whether he even went to some of the faraway places he has grandiose tales from, and it was even harder for the contemporary reader to find out for himself.

Thucycides wasn't the only one to 'slander' him though, Cicero, and Plutarch, later Romans, also claimed there were a lot of legends in Herodotus' work, and from Plutarch he obtained the nickname 'The Father of Lies', funny enough.

It's not all bad though, he did make disclaimers about whether or not what he was including was simply 'what he heard' (and you have to take his word for the veracity of his factual claims quite a bit anyway) or whether it was stated as pure facts, so take for it what you will. As it is with any primary/secondary sources, use several different ones; Don't lean too heavily on one author or book, and expand your view to make sure you don't run in to any biases. Herodotus had biases, so did Thucydides, we all do, and we'll angle our story-telling to fit our own narrative. :)

Thucydides' criticism of unreliable re-tellings of the war is worth reading though, the following excerpt is from Thucydides' History of the Peloponnesian War 1.22:

What particular persons have spoken when they were about to enter into the war or when they were in it were hard for me to remember exactly, whether they were speeches which I have heard myself or have received at the second hand. But as any man seemed to me that knew what was nearest to the sum of the truth of all that had been uttered to speak most agreeably to the matter still in hand, so I have made it spoken here. [2] But of the acts themselves done in the war, I thought not fit to write all that I heard from all authors nor such as I myself did but think to be true, but only those whereat I was myself present and those of which with all diligence I had made particular inquiry. [3] And yet even of those things it was hard to know the certainty, because such as were present at every action spake not all after the same manner, but as they were affected to the parts or as they could remember. [4] To hear this history rehearsed, for that there be inserted in it no fables, shall be perhaps not delightful. But he that desires to look into the truth of things done and which (according to the condition of humanity) may be done again, or at least their like, he shall find enough herein to make him think it profitable. And it is compiled rather for an everlasting possession than to be rehearsed for a prize.