But every programming language I've ever seen has flaws - that are both objective and subjective. If I were a Ruby developer wanting to create something on the JVM, I might think that groovy is inferior to JRuby, for example. If I were a straight up Java developer, I might think that groovy is slow or inconsistent or too unsafe (how some people perceive ruby's monkey patching). I'm not saying any of those perceptions are correct, but it is very easy to imagine all kinds of reasons and perspectives for disliking every single language up there. Heck, for about a third of them, it's harder for me to imagine reasons that people DO like them. But I know that's just my opinion, man, and who I am to stop people from using drugs, jumping off cliffs, or using Perl.
Of course, there's the limitation that it only runs on the JVM. Maybe it suffers from the following thought process: many people who want a rapid development language and would be interested in Python or Ruby aren't interested in running on the JVM, and people who are already in the Java ecosystem are too biased in favor of static typing to give Groovy its fair shake?