[octygen]

Until mermaid comes up with the vast list of diagrams one can create in Plant UML, I don't see it as operating in the same weight class. E.g., wireframes, basic Gantts, AWS diagrams, component diagrams with graphviz (thought I confess I hate this dependency), archimate diagrams if you're really hardcore.

[ausjke]

for UML it seems mermaid is indeed good enough, I used mermaid but never used it for OOD UML, after a quick read it looks very simple and straightforward for UML comparing to plantuml, especially you can embed mermaid right into markdown as text