This isn't a Cesium problem per-se, but this seems to be the best place to ask so please bear with me.
I've been writing a variety of primitives for the Live Earth web app (which uses Cesium) for things like animated traffic, weather, heatmaps, billboards, etc. Generally this has been a pretty smooth process until the point where I test on an iPad pro. Almost every shader has taken longer to get to work on iPad then to actually develop in the first place, and some still don't work on iPad for reasons that are mysterious to me.
Generally the problem manifests as nothing being rendered on the screen, whereas on a windows desktop in Chrome the primitives render just fine.
Here are some of the issues I've encountered so far...
1. Using multiple VertexBuffers in a single VertexArray (one for static, one for dynamic, and one for instance) causes one of the static or dynamic vertex buffers to be ignored and the other to be written to the wrong vertex offsets/attributes.
2. Having an unused vertex attribute in a shader causes that vertex attribute to be optimized away and the buffers then are written to the wrong offsets/attributes (this is only typical during development of the shader)
3. Adding too many uniforms (about 5 more than the automatic Cesium uniforms, I don't remember the exact count) causes the shader to not render even though the number reported by webglreport to be supported is much higher (128 vertex uniforms are supposed to be supported, but about 16 are used in practice even counting for mat4 and arrays using extra slots)
4. ?!?!? Still puzzling out some baffling issues.
The builtin primitives in Cesium seem to work just fine, so I assume someone on your team has worked through a lot of these issues and may be able to give some tips.
Finally, my questions - What other issues have you encountered writing the primitives that ship with Cesium on iOS? What tips do you have that would help narrow down issues faster? Any resources for further reading? I have read http://codeflow.org/entries/2013/feb/22/how-to-write-portable-webgl/ and http://codeflow.org/entries/2014/jun/08/some-issues-with-apples-ios-webgl-implementation/ by the same author.