Between 1988 and 1992 I worked for a UK company participating in a multinational project to use Common Lisp to build an expert system building tool. After creating the thing, we worked with clients (internal and external) to try to solve real customer projects. Our conclusions matched others, and contributed to the AI winter:
* the rule-based expert systems were extremely brittle in their reasoning
* once you got beyond toy problems with small rule sets, you needed some programming skills in addition to the domain expertise that you were supposedly encoding
* you sometimes spotted the actual algorithms that could be coded in a conventional language rather than rules + inference engine.
We eventually abandoned the AI side and kept going with the underlying Lisp, and started to get real leverage in the company, rapidly becoming a prototyping / risk reduction group who delivered usable functionality.
[Edit] We were using Lisp processors embedded in Macintosh hardware, with outstanding (for the time) IDEs and thanks to the Mac interface, we could create some really slick apps for the end users. One of our Lisp systems that got rave reviews internally was a drag-and-drop network modelling tool that replaced a frightening mass of Fortran and data entry spreadsheets. No AI/ML at all, but it really improved the throughput of our network modelling group. As we were a comms company, this got positive reaction from senior management, offsetting the non-return on investment in the rule system.