I would not say shootouts are completely useless, but they are flawed unless the cables and other components are optimized for each DAC (or other components of a shootout).
The wrong cables will kill a component's sound. I've done this many times; taking my best sounding cable for one component and put them on the new component, only to get terrible sound. This also applies to Transport-DAC combinations, Amp to speaker combos, etc.
Many DACs sound better with a really good preamp, even though they can be connected directly to the amps. Put a different DAC in, and it may sound better without the preamp.
So the people doing the shootout, really have their work cut out for them, if they want to be thorough.