If a cultivator reached god state peak stage and continued in body refinement, the body wouldn't be able to take it. At this moment, the body refinement cultivator would face his greatest test, that was nirvana. Only after nirvana would the body go under fundamental evolution and become an immortal nirvana body.
So, nirvana was extremely important for body refinement cultivators. Many body refinement cultivators failed nirvana and forever regretted it their entire lives. After nirvana failure, a light consequence would be that they couldn't practice body refinement anymore and could only focus on dao cultivation. The serious consequences would be that one would only have an essence spirit left. Even with an immortal essence spirit, they would have to find rare treasures to rebuild their body. After that, it was pretty much impossible for body refinement again.
