The creaking and spider cracks on the gen1 DA9000 cassettes are unrelated to one another. The creaking is from Ti cogs and the original 9000 chain surfaces not playing nice. The new HG900 chains have remedied this issue with increased Sil-tec treatments.
The carbon spider cracks seemed to be isolated to certain riders having multiple issues rather than single instances over many riders for whatever that is worth.
I wouldnt hesitate to buy a 9000 group ( I have a few
)