I had a project where we wanted a ticketing / events ontology and budgeted 6 weeks for three people to build it, in the end we spent probably 3 person years on

I had a project where we wanted a ticketing / events ontology and budgeted 6 weeks for three people to build it, in the end we spent probably 3 person years on it, which was dim... but we got suckered in by the idea that the ontologies themselves would be valuable (spoiler - nope). So, knowledge engineering scales badly, but there were other problems. There was a big debate in the EU community about what kind of reasoner to use, and for some god awful reason F-logic was chosen, at the time we thought that reasoners like Otter wouldn't be able to scale and do FOL tractably. It's a shame that answersets and MCMC probalistic reasoners were 10 years later - I think that the weak reasoning and poor representation systems were big gaps. The other problem was institutional, the way that EU semantic web funding worked, and the way that the projects developed. A lot of money was spent, and then there was no money - there was no self sustaining legacy.