It is usually said that Planck units have no scientific ground, yet they are useful indeed because many laws collapse, make no more sense at, say, Plancks length or time.
Can you mention a couple of laws that support this opinion and what makes no sense at a length less then $1.6 \times 10^{-35}$ m?
We have, at yet, no specific evidence that any of our current laws of physics collapse at the Planck scale. However, we do expect that at or near the Planck scale quantum gravitational effects will become important. We don’t know which ones will break down nor do we know in what way they will fail.
None of this has anything to do with “justifying” Planck units. Units are just defined, their definitions do not need justification. Planck units are simply defined so as to make many constants equal to 1. No further justification is needed.
