Most platforms that at least support OpenGL/ES seem to be at least similar enough conceptually that you should just need different optimised render paths, not a

Most platforms that at least support OpenGL/ES seem to be at least similar enough conceptually that you should just need different optimised render paths, not a totally different engine. Different render paths are something you'll need anyway, regardless of API, unless you're targeting exactly one machine. You'll probably struggle more with the difference in RAM and CPU power available to the app than the different GPUs. In the simplest case you just use bigger textures and higher-poly models on better hardware if they all implement GL ES. Compare this to, say, the Nintendo DS, which is different from any other GPU I've ever worked with; writing a cross-platform engine that supports it is tricky. There isn't a GL ES implementation for it as far as I'm aware.