The biggest concerns are the lack of thought put into build tooling and breakage of existing code. Jigsaw as it is now seems to have been primarily designed to

The biggest concerns are the lack of thought put into build tooling and breakage of existing code. Jigsaw as it is now seems to have been primarily designed to split the JDK up and was pushed into a language-wide module system without enough thought put into the requirements for such functionality. I don't think anyone is against JPMS solely because they prefer OSGI, etc., but because it can require dramatic changes to the way these systems work (if it can even keep them working at all). Java 9 will likely ship with Jigsaw in some fashion, at worst it will be limited to the core JRE/JDK libraries themselves.