It was, in a nutshell, not great: there's a fundamental contradiction between wikis, which are very loosely structured and edited by humans who do all sorts of wacky unpredictable shit, and hierarchies, which need to be carefully pruned and maintained.
So this use case didn't work, and I think it's telling that (AFAIK) not a single Wikimedia project has adopted it.
SemanticMediaWiki has some questionable scalability when it comes to large sites. Wikimedia was never going to adopt it for technical reasons.
The advantage of SMW imho is still that it brings the semantic / structured data features right into the MediaWiki itself and that there's a big ecosystem of related extensions around it (like Page Forms). So far I don't know of an open-source information management system which comes close in capability to this. Setting this all up and maintaining it is difficult, though - I even had to start developing custom tools to assist me with that once it got more complex.
However if we're talking about Wikipedia, any scaling concerns at all basically make it a non-starter.