Bhagavad Gita, Chapter 17, Verse 18

That austerity which is practised with the object of gaining welcome, honour and worship, and with ostentation, is here said to be Râjasika, unstable and transitory.

Even real, disciplined effort is shown here producing only unstable results once the motive is prestige.

Krishna names the actual outcome directly, benefits that don't last, tied to a motive that was never aimed at anything lasting either. Real instability matches a real, shallow motive.