I really like Mermaid, and use it quite a bit (people forget how long they spend dragging boxes, arrows, aligning in Visio/DrawIo), but for some diagram types, you have no control over layout, there isn't even hinting (yet - it continues to improve).

Sequence diagrams are great. I imagine Gantt charts are fine. Flowcharts will sometimes be rendered unintelligibly.. adding/removing a link or new element will completely rearrange the layout. If you have any implied order of your flow (as simple examples on the website imply) this maybe rearranged as your flow gets more complicated. If you want flowcharts over swim lanes (either distinct or implied - think of data-processing ingress on an app), or the start point first and the end point last - this isn't a great tool. There is support for subgraphs, but you have no control over their location either.. so if you want a web-firewall-loadbalancer-app-db type flowchart, it may be rendered in a circle.