Many people swear by them.
I like that Luna has a text syntax.
I also like that Luna supports building graph functional blocks that can be nested inside other graphs. That's a missing link in other tools of this type that limits the scale of what you can do with them.
I'd like to see a tool like this which has something like a JSON document going over the lines, but that might be asking too much. If the objects going over the lines are basically relational rows you can do columnar execution at very high speed. The Actian people have looked at alternatives to what they do, but I think they haven't extended the data model past relational because they couldn't get it fast enough to compete with their own product.
(e.g. a classic "disruptive technology")