1. No good way to embed the diagram source in my markdown, as a readable format. I mean, I could just paste it in and comment it out. mermaid.js has a browser plugin that allows it to be rendered on github.
2. Not a 1 stop shop for UML, I personally use sequence and state diagrams a lot too, I know I can use other products, but then I might have to pay for them too, and it adds up, especially at $5 a month which is $110 a year, which is close to the price of something like Astah Pro. If I have to pay the same for additional products it quickly becomes unworth it.
3. No library import support, by this I mean, I can't say, hey this is a Year object from java.time in multiple diagrams, so it gets repetitive, and huge.
4. Another point on note 1, not really a great way to commit these diagrams to source, git isn't supported.
I do like the syntax though, far more intuitive than mermaid, imho. Just my feedback, do with it what you will.