For all that classic dmus were frequently pilloried, they did provide a substantial amount of service for more than a generation.
Some things seemed to be imposed by BR, and some left to the individual builders. Broadly anything beneath the underframe was standardised, power train, Blue Square controls etc, and the bodywork above was up to the designer. Some seem to have had Region input, such as interior colour schemes corresponding to their sponsoring region.
I wonder what it was about Cravens designs that got them such a poor reception. Their 100 single-engine but higher powered cars were indeed plagued with problems, but it must have been a BR idea to deviate from the symmetrical norm. The rest of what they did was mechanically standard, they built in steel instead of alloy, but they just seemed lacking in interior ambience.