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.

PlantUML offered more decade ago. Marmaid is a toy compared to it.

However, since its so powerful, its hard to write. Actually, any given diagram is not that hard, but to keep all those syntax rules in head is IMO almost impossible.

They are quite similar, PlantUML has support for more types of diagrams than mermaid (such as network diagram), but IMO PlantUML is "messier" to write, I find Mermaid more elegant in writing diagrams.