It's one of the few books that is structured in the same way a Scala developer thinks. Lots of Scala books follow the same order: keywords, usual syntax, features also in Java, features exclusive to Scala. I think this is damaging for any person looking to learn Scala, because it does nothing to lay out the way the features interact together. for instance, the Oreilly book shows pattern matching and implicits before class hierarchies, and that maps well with how Scala devs work, because the former features are encountered daily, while the latter tends to be uncommon.

As for maintainability: you're right. Stricter languages are more maintainable. There's also the issue of powerful languages attracting certain kinds of people. Over the years, I've settled on a simple policy: reject everything that is not documented. This goes for both internal code, and external dependencies.