ah, I mean, working in the sense of OP: that a system which overcomes the "engineering hurdles" is actually feasible and will be successful.

To be blocked merely by "engineering hurdles" puts QC in approximately the same place as fusion.

There are working quantum computers that are not only feasible, but exist, can be rented on the cloud, and are people pay money to use.

Whether these are a commercial success at this point in time is missing the forest for the trees. A LOT of money has been put into getting as far as we have, and the limited market for using these machines at the moment means that getting a return on investment right now is difficult. But this is/has been true of every new technology.

And quantum computers are getting better & more energy efficient year-by-year.