Perhaps PlantUML isn't as new and interesting as Mermaid, but each time I try a side-by-side comparison, the results are always better in PlantUML than Mermaid. I'll admit that the Mermaid syntax is a lot easier to understand, but the PlantUML syntax is a lot more powerful. I especially like the new Activity diagrams in PlantUML with the conditionals and looping syntax.

Thats what I also found out, supported by llms, nice looking output. I have not found a proper left to right renderer for the activity diagrams

Agreed.

I use it mostly for sequence diagrams, but never hit a case where Mermaid felt better than PlantUML. It has been more of a forced transition as SaaS like Notion don't have a PlantUML integration or it's not available at the organization level.