[lostcolony]

Finding Scala developers vs Java developers.

That's really the main issue per the original question. It's not that functional somehow fails on its merits to deliver, it's just that "at scale" means having enough developers who know it. The reality is that imperative/OO languages have the greatest mindshare, and thus it's far easier to find developers, and far less wind-up time, if you're using such a language.

[moomin]

Also, there's enough different about Scala that can't be expressed on a simple OO vs FP axis that it's hard to learn that much from Scala vs Java. (On this axis)