The folks over at Imaging Resource tried to make the A9 overheat in very unfavorable conditions:

It is extremely challenging to overheat the A9. I say that as even though I was actually trying to make the camera overheat, I could not.

So we can put the “a9 overheating” story at peace. It also seems like that few of the testers who experienced the overheating in extreme conditions were also using slow SD cards which of course does keep the processor running for a long time to clear the buffer.

Therefore, if you buy the A9 make the wise choice to buy fast cards too:
Sony SF-G 128GB: BHphoto, Adorama, Amazon. In EU: AmazonDE, AmazonUK, AmazonFR, AmazonIT, AmazonES.
Sony SF-G 64GB: BHphoto, Adorama, Amazon. In EU: AmazonDE, AmazonUK, AmazonFR, AmazonIT, AmazonES.
Sony SF-G 32GB: BHphoto, Adorama, Amazon.

Sony A9:
Sony A9 at BHphoto, Adorama, Amazon, BestBuy. FocusCamera. Calumet.de. Park UK. Wex UK. Jessops.
Sony A9 with Grip Extension Kit at BHphoto, Adorama and Amazon.

DxOmark tested the sensor and cam to that conclusion:

The Sony a9’s sensor ranks as one of the best we’ve tested at base ISO, with consistently good scores for dynamic range and color depth, and exceptional results for noise. At those crucial mid to high ISO sensitivities for action photographers, image quality is also excellent and on par with flagship Nikon and Canon sports DSLRs.

For the very best image quality, the A7R II offers better results for color, as well as almost twice the sensor resolution, but it isn’t a sports camera and can’t compete for performance against the a9, so they’re intended for different markets.

With a big body-only price tag of $4,500, the Sony a9 is squarely in the pro category, going head-to-head with the top-end Canon and Nikon DSLRs. With many pros already heavily invested in lenses and accessories for those systems, it’s going to be a tricky (not to mention expensive) to switch, but the Sony a9 has both the performance capabilities and image quality that make it a seriously tempting proposition.

Like I told you before you have to compare apples with apples. The A9 is made to satisfy the Image Quality needed by Sports Photographers and not Landscae shooters. If you compare the A9 against the more expensive competition the A9 does beat them:

Sony A9:
Sony A9 at BHphoto, Adorama, Amazon, BestBuy. FocusCamera. Calumet.de. Park UK. Wex UK. Jessops.
Sony A9 with Grip Extension Kit at BHphoto, Adorama and Amazon.

And the fastest SD cards are now in Stock:
Sony SF-G 128GB: BHphoto, Adorama, Amazon. In EU: AmazonDE, AmazonUK, AmazonFR, AmazonIT, AmazonES.
Sony SF-G 64GB: BHphoto, Adorama, Amazon. In EU: AmazonDE, AmazonUK, AmazonFR, AmazonIT, AmazonES.
Sony SF-G 32GB: BHphoto, Adorama, Amazon.

The new Sony FE lenses preorders:
16-35mm GM at Bhphoto, Adorama, BestBuy, Focus and Amazon. In EU at Calumet DE, WexUK, Jessops.
12-24mm G at Bhphoto, Adorama, BestBuy, Focus and Amazon. In EU at Calumet DE, WexUK, Jessops.
Sony 100-400mm GM at BHphoto, Amazon, Adorama, BestBuy. Calumet.de. Park UK. Wex UK. Jessops.