My take on it: you get "infinitely parallel" computation, generating "inifinetly parallel" results that are in superposition. Problem is, at the end you can only access your "infinitely parallel" result via a classical measurement, involving a wave function collapse [1].
For some problems, people have found ways to extract useful information via classical measurements, e.g. Shor's algorithm (in theory breaking RSA/DSA/ECDSA/DH/ECDH style public-key algorithms [1]). However in the general case this does not work (so AES and hash algorithms are safe for now).
I mean that's pretty much it. You can't use a quantum computer like a classical computer with a huge (infinite) number of compute cores. Its not a gpu. That's not how it works.
If you use the metaphor that superposition is like computing many things in paralell, the problem comes in that when you measure. The superposition collapses to a single answer at random (with probability related to the amplitude of each possibility) which will usually not be the answer you're interested in.
For some problems, people have found ways to extract useful information via classical measurements, e.g. Shor's algorithm (in theory breaking RSA/DSA/ECDSA/DH/ECDH style public-key algorithms [1]). However in the general case this does not work (so AES and hash algorithms are safe for now).
[1] https://en.wikipedia.org/wiki/Wave_function_collapse
[2] https://en.wikipedia.org/wiki/Shor%27s_algorithm#Quantum_par...