In quantum mechanics, there are many problems; the biggest of them (according to a group) is finding the first and initial state. We are talking about the initial state of the observable universe; it sounds cliche. But people believe that if we can find the initial state, we can determine the difference between the macroscopic and microscopic world. I have not any thoughts on that. However, as like many, my respect for quantum mechanics is just fundamental (and theoretical). When you do quantum mechanics, without any evidence of research, you will likely come to believe that 'what is true and what is wrong' is differently to be seen in the subject.

Just like the statistical mechanics, we have $\rho$ the density matrix in quantum mechanics. This looks like

which tells you the probability of each state into finding, the bracket notations used in this style is called projection operator. If you are a realist, you should not see this as a superposition, and in fact, it is not any superimposed thing. This $\rho$ being classical shouldn't be the problem. 

When we come to 'Copenhagen's Interpretation of quantum mechanics, which is heavily misguided in different ideologies, we can observe incompleteness and loss of determinism. The former and latter can be written to co-exist as 'no macroscopic evidence.' It is not entirely true. However, it is accepted that the interpretations can be completed by the completeness of matching the probabilistic theory to macroscopic lurches of evidence. Indeed, we would enjoy a version like this. Rather than this, the Copenhagen interpretation is widely celebrated as a successful interpretation.

_____________________

The reason these are half-baked problems that no one now cares about it. These problems are widely accepted as 'reality and fundamental.' Even when young people try to think about it, they fall into the prey of 'cancellation' and 'no go' which they say are occults of the subject. But, my stance is quantum mechanics is an achievement that we should sing from time to time.

Notes; A friend of mine shared with me an unlisted link of a talk by Weinberg at 'Standard Model at 50', in which he talks the same. And in fact, this post is inspired by his talk. 

https://www.youtube.com/watch?v=mBninatwq6k