In the past I used to use Mermaid for simple (but not trivial) flow charts and class diagrams.
I've found some minor rough edges, but in particular, a couple of missing features, one of which I needed (edges going from attribute to attribute in class diagrams).
PlantUML didn't have any of those limitations, so I switched, and never looked back.
For simple-but-not-trivial diagrams, I didn't find any substantial difference in terms of syntax (complexity). I don't have experience on complex diagrams, though.
I've found some minor rough edges, but in particular, a couple of missing features, one of which I needed (edges going from attribute to attribute in class diagrams).
PlantUML didn't have any of those limitations, so I switched, and never looked back.
For simple-but-not-trivial diagrams, I didn't find any substantial difference in terms of syntax (complexity). I don't have experience on complex diagrams, though.