I've only used the nRF5 SDK. It worked, and it worked well, but it was cumbersome to work with if you didn't want to keep your code in the same tree as the the SDK itself. This was probably an organizational issue on my part, honestly.
The other thing which really bit me was someone here used a symbol name which collided with one used in the SDK, but the build system allowed this to link without error.
The other thing which really bit me was someone here used a symbol name which collided with one used in the SDK, but the build system allowed this to link without error.