Can a quantum computer only perform certain very specific tasks,
I am afraid so.
Most types of tasks done by a conventional computer wouldn't be done faster by a quantum computer.
For example, if you just want to add or divide etc two numbers then even your ordinary pocket calculator would do the job better than a quantum computer.
In addition, and only with just one exception I know of, there has been much hype not corresponding to reality of most of the kinds of tasks that a quantum computer CAN do faster because all but one of those types of tasks would have only trivial (if any) benefit to humanity.
The only exception I know of to that is the task of simulating quantum systems; there IS a big potential benefit from quantum computers doing that partly because that should indirectly tend to lead to huge breakthroughs in material science esp for designing the materials solar cells of solar panels and such things as high temperature superconductors and designing materials for spintronic microchips that would be over a thousand times more energy efficient than our current microchips. Also, quantum computers simulating quantum systems should give us much better understanding of how enzymes work and that should lead to medical breakthroughs as well designing better artificial enzymes for various important industrial processes which could be done more energy efficiently and with less capital costs with enzymes.