Same experience here.

I settled on Fable for design, Opus for build routine, and every now and then I'd try out the Chinese models. In my experience they do fine on small codebases, and quickly get confused on anything larger than 500k LOC.

My use case is: mature, very well documented, fully Ai written code, with about 1:10 ratio of prompts/specs to code, and codebase sizes >500k and <2m LOC. Once one achieves the state of good, comprehensive design documentation I can literally vibe code with single sentence prompts thanks to the extensive test coverage, ADRs, and tens of thousands of lines of specs. Fable/Opus works predictably well, the Chinese models are literally dangerous to the codebase.