As far as I know people do not doubt D-Wave's machine is a quantum computer because it is adiabatic instead of the general kind, but because there are doubts that it speeds anything up at all compared to normal computers. If it doesn't, then it might not even be an adiabatic quantum machine.

How is it not clear which is more performant?

You can read more about that here: http://phys.org/news/2014-06-independent-group-d-wave-quantu...

Scientists claim is that a previous positive comparison was unfair because, "problems designed to run well on the D-Wave Systems were run without optimizing on conventional machines"

Short version: It's not a universal quantum computer, but it's a device using quantum mechanical effects to solve a specific type of problems faster than conventional computers can.

I hasn't been proven to be any faster than conventional machines under fair testing. It is unclear that it has any algorithmic advantage as of yet.

I think it takes a broader definition of "computing" to really answer the question in the end ...

Betteridge's law of headlines strikes again.

It's like what teachers and cops tell you in elementary school to do if someone offers you drugs: "JUST SAY NO".