Oh god the horror that was OpenCL on Xeon Phi.
1) It was extremely buggy. Any OpenCL program of decent complexity was bound to encounter bugs in their driver.
2) FLOPS are not the only measure for computations / simulations. The bandwidth is is extremely important for many applications. Whatever Xeon Phis OpenCL was doing was not achieving anything close to its peak. Any OpenCL kernels we had tuned would end up improving the performance of the host CPU (16 x 2 Xeon) as well. It would usually result in the Xeon CPU performing better than the Xeon Phis.
3) NVIDIA released multiple generations of their GPUs since Xeon Phi was released. The performance per watt on NVIDIA and AMD GPUs has been improving substantially over the last 3 years while Xeon Phi's stagnated in 2012.
4) Even their claim for best FLOPS / WATT is wrong. The Tesla K20 released around the same time had the same power usage (225W) but about 50% higher FLOPS (2TFLOPS vs 3TFLOPS single precision).
The only selling point was that they were x86 cores and hence did not require rewriting your code. But you had to write additional directives indicating what part of your code you wanted to offload to your accelerator and when you wanted to get it back. The best performance ironically came after a lot of tuning for your application.
4 years down the line, GPU based accelerators still dominate the market and growing. and I wonder how many of the researchers who went with Xeon Phis regret their decision.