I have seen PLENTY of divers with C cards whose skills completely sucked,I think it all depends on your instructor and your level of dedication.
Speaking purely about higher-level/advanced qualifications, which I consider solo diving to be, there shouldn't be too many divers around whose skills suck. Likewise, the quality of instructor should represent the level of diving which is being taught. There are obviously occasional disappointing exceptions to that assumption.
Most agencies that provide solo diving training demand that the instructor holds tech-level teaching qualifications as a minimum prerequisite. That should go a long way to ensure a higher degree of expertise and more robust definition of 'skill' is applied to the training and assessment.
And you can assess your self taught skills against other divers that you observe,
The point remains - what and how do you perform that self-assessment? How do you know if/what you might miss from the assessment?
Do you get to observe many peers dealing with critical failures on a regular basis? If not, how do you assess yourself against their ability to deal with those uncommon scenarios?
A capable advanced-level diving instructor
(that's the olde school meaning of 'advanced') will have a myriad of ways to put the student under realistic pressure to see where the breaking strain is...and how their performance suffers in the worst case scenario. Such assessment cannot be equaled by conducting a hypothetical comparison with peers under more optimal circumstances. That's exactly the sort of 'flawed assessment' to which I referred earlier.
You cannot 'qualify' yourself as a competent solo diver based upon a finite appreciation of your ability under the best case scenario. In that, I am sure, the analogy to firefighting is very close.