I've build a few Semantic MediaWikis in my past and share the concern about scalability. But also be aware that in many cases you might not need such a scalability like managing semantic data for all of Wikimedias major wikis in different languages. For small to medium sized companies or communities, I think SMW can scale well enough - depending on how you use it.

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.