Frankly for all it's issues, plantUML has been the most customizable and flexible for me. Mermaid is fine for diagrams where you don't need to customize the look and feel much; it starts to become a pain once you need to. I just wish Obsidian had as a good a support for plantUML as it does for Mermaid.
I kinda agree, and I mostly switched to mermaid because of availability. Plantuml is much more annoying to setup, and doesn't render natively on GitHub (last I checked, anyway), so if I writing anything more collaborative, mermaid requires a lot less explanation or help with setup.
And for me, one mediocre tool that works everywhere for everything beats a better tool that requires me to context switch too often, so I mostly retired Plantuml. Not happy about, but alas.