Novag did not compete with Mephisto and Fidelity in terms of the strength of their chess computers. They used cheaper processors for their top models and the same programmer virtually throughout.
By the time that the 16 bit Diablo replaced the 8 bit Super Expert as their top auto sensory wooden board, Fidelity had been using 32 bit processors in their top Elite Avant Garde models for two years, and so had Mephisto in the Portorose then Lyon modules.
The Diablo cost £599 when it was released in late 1991. It was competitively priced with the Mephisto Exclusive Dallas 68000 which cost £629 at the time. However the chess computer market was shrinking. Not many Diablos seem to have been sold. This may also have been due to distributor and pricing difficulties in the USA. As I recall Diablos were not marketed as strongly in the UK as the Super Experts they replaced.
Today the Novag Diablo is a much sought after model with only two or three a year on average finding their way onto Ebay.