Tee hee. I have a fine counterexample.
About 15 years ago, my company (a producer of VMEbus and CompactPCI boards) designed a video module. We used a Trident mobile graphics chip. Unfortunately, we were attempting to use it with a PowerPC, not an x86 CPU. We had the big user manual for the chip, but when we programmed all the registers according to the published configuration info, it refused to initialize.
We then were given the BIOS object code from the factory (they wouldn't share the x86 assembly source code). We disassembled the code. It was such a tangled web of spaghetti that we never did figure out how to get the part initialized, and the factory app engineering team was unable to tell us how to do so either.
We eventually dumped the part and used an Intel part with C source code available. It worked just fine.