The implementation is ingenious and very easy to use. Pushing from a GIT is both elegant and effective.
However, this exposes the entire code base to Heroku.
That is troublesome for anything but an OSS application.
Heroku started as a Rails platform where there is no alternative to deploying the source code. That is not true for Java or Scala.
Obviously this disadvantage does not greatly impact the business model of Heroku or their competitors. Presumably their customer base is not overly concerned with IP protection