Bhagavad Gita, Chapter 17, Verse 5-6

Those men who practise severe austerities not enjoined by the Shâstras, given to ostentation and egoism, possessed with the power of lust and attachment, torture, senseless as they are, all the organs in the body, and Me dwelling in the body within; know them to be of Asurika resolve.

Real, severe effort is named here as not automatically good, even genuine conviction can be misdirected, right down to torturing one's own body, which is described as a real offense, not a neutral personal choice.

Krishna doesn't credit intensity on its own, he names the real motive, hypocrisy, ego, behind these harsh practices as the actual problem. These practices disrespect something genuinely present within the body being harmed, so effort without proper direction doesn't earn credit here.