How many Direct/Compute/Copy Queues are meaningful?
Ship your application with a benchmarking sequence testing the actual platform. (Possible answer for many questions I guess...)

I suspect the performance is highly dependent on how you use the hardware. Since the hardware is unlikely to somehow instrument your application backwards, telling you what to do, I´d go with whatever looks good in your design.


  "...command queues can be executed concurrently by the device..."


Keyword is CAN. I see no reason why any vendor would screw this up. In the end it is the platform provider (Intel/AMD/Nvidia) who is responsible of making you a good enough driver for you not to consider switching vendor. If they do have a "know issue" with this functionality (which by the way has no functional meaning, only performance) then they should also solve it using what they know. I mean for crying out loud, the fallback is something they have already implemented; synchrounous execution. 

Hardware is enough voodoo as it is for us developers.