I feel like Atom cores get too much of the blame for Intel mobile failures. Yes they suck compared to big core, but back when they FIRST were competing in the mobile space (2011-2012) they were ahead of what ARM provided. The XOLO X900 blew away ARM phones on single core performance, the problem was the GPU was far far behind the top of the line phone SoCs of the time.
If Intel could have counted on its GPU division they maybe could have gotten some early design wins, but instead they had to lean on PowerVR GPUs (like EVERYONE else had) because their internal stuff couldn't cut it. THAT is what killed their chance in mobile.
People love to bash Atom, but the Intel GPU division has wasted WAY more money than Atom ever could. Hell, aren't those Knights Landing micro-cores or whatever they are based on the Atom?
Man I don't know.. I stuck up for Saltwell being not-that-bad when people bashed it but I also put it down as not-that-amazing when people said things like that. At the time a lot of benchmarks being ran were really x86 friendly so it looked spectacular in reviews. Then there was that big AnTuTu stunt that even got an Intel-sponsored "analysis" written to show how utterly incredible Saltwell was.
This was especially true in Javascript benchmarks where ARM code generation in Chrome was still behind (and in some ways maybe still is, look at Chrome vs Safari). This in particular says a lot:
http://images.anandtech.com/graphs/graph8560/67833.png
First came Samsung Galaxy S4, April 2013, 1.6GHz Cortex-A15, 1183 in Sunspider. Then came Huawei Honor 6 August 2014, 1.7GHz Cortex-A15, delivered 645.
In actual real world programs that were decently optimized about the same for ARM and x86 Saltwell was a little behind Cortex-A9 and Krait in perf/MHz while offering similar MHz to Krait (and a little more than A9s) so I don't think it really blew away ARM phones in single core performance. And then there was the whole matter of relying on binary translation for Android apps that didn't support x86 natively, something which destroyed performance. Intel brushed this off by saying ~80% of Android apps didn't even use the NDK, but that was certainly not the case when weighted by app popularity.
Also, Medfield was single-core with HT while ARM phones were already quad-core. So in anything even moderately threaded it hurt pretty badly.
GPU-wise I fully blame Intel, not PowerVR. For a long time Intel chose very very small/conservative GPU offerings. When Apple used PowerVR GPUs of the same generation they went with much more robust configurations and smoked Intel.