I've found old SDK actually not that bad. Documentation is not great, but sources are available and it's just mostly C code with some hairy macros, but it's manageable. After few weeks I became quite proficient. I don't like their suggested approach "copy&paste example you like and tinker it", I never do that, so I had to untangle their Makefiles, I had to research their defines (app_config.h, what's a monstrosity), I had to write my own linker scripts, but in the end I have my application under control and it's all standard gcc toolchain, no fancy stuff.