Criticizing Vedas is a sin that is equal to one of the major sins of Hinduism viz:- drinking Sura.
Manu Smriti 11.56. Forgetting the Veda, reviling the Vedas, giving
false evidence, slaying a friend, eating forbidden food, or
(swallowing substances) unfit for food, are six (offences) equal to
Whether criticizing is a sin must be depending on the context when it was done, but Manu mentions it as one of the vices particularly for the King.
7.48. Tale-bearing, violence, treachery, envy, slandering, (unjust) seizure of property, reviling, and assault are the eightfold set (of
vices) produced by wrath.
A disciple criticizing the Guru, even if it's "fair", is severely condemned in all Hindu scriptures, including the Tantras.
Manu 2.201. By censuring (his teacher), though justly, he will become
(in his next birth) an ass, by falsely defaming him, a dog; he who
lives on his teacher’s substance, will become a worm, and he who is
envious (of his merit), a (larger) insect