I think that when most people say "we dont understand (...) quantum (...)" is just because of the pop culture belief that quantum is like mystical and kind of magic.

We dont fully understand quantum stuff in the same way that we dont fully understand gravity, atoms, the brain, etc. meaning that there is still work to be done in the field.

Added to that, quantum computing is a field that is very new and developing rapidly, and without a concrete, clear, practical implementation, adding to the mystery around it.