Graphviz is hard. I only need a graph making tool three or four times a year and when I go back to mermaid, only 5 minutes of going through the documentation get's me up to speed. But graphviz is much more complex in a way I often don't need. It's also pretty verbose; You first need to define nodes then the connections while in mermaid both are done in a single line.
However mermaid's experience and output is definitely subpar. Under the saved graphs section you find randomly saved graphs and there is no way to organize multiple graphs in the web editor.
I've even thought of writing a simple script to translate mermaid charts into dot language.