In practice though, the familiar syntax and semantics often leads you to believe you can do things that you in fact cannot, until you face the "this feature is

In practice though, the familiar syntax and semantics often leads you to believe you can do things that you in fact cannot, until you face the "this feature is not supported" error or go read the documentation and search through the long list of unsupported features, then curse yourself and rewrite your code around the lack of said feature. It's the gap between expectation and reality, coupled with the productivity hit of often using some feature, then having to rewrite without said feature, that leads me to dislike subset languages. It's often ultimately a different language with only syntactic similarity.