"There were some problems with Scala in the real world. Compared to Java it had a big learning curve, what with all the features it took from Haskell and functional programming while also keeping all the old features of Java. Some of the libraries weren't as mature as their Java equivalents or there were two equal competing libraries whereas with Java there was just one popular library for a given thing. Hiring Scala developers was an issue. Compile times were bad. Every major version of Scala (ex. 2.10, 2.11, 2.12, etc.) broke backwards compatibility so if you had something with say Scala macros that compiled in Scala 2.10 it wouldn't necessarily compile in Scala 2.11 without code changes. We had a library where the maintainer abandoned it at Scala 2.10 and we couldn't upgrade to Scala 2.11 without either taking the library out and using something else in its place or taking over the library, which just wasn't feasible. There was an issue with Scala that I've seen with C++ as well where the language is so big that different teams and different people program in different "dialects" of the language, using different subsets of language features (ex. some people used OOP class inheritance and some did not). Lots of people didn't get Monads or functional programming. But yeah, lots of places ended up going back to Java or using Node.js if they wanted to be async/reactive. "