Even though PlantUML has the abbreviation UML in its name, many representations are not UML-compliant!
For example the decision nodes in the activity diagram. With uml, the condition is not modeled in or at the node, but at the outgoing edges as a guard.
There can be any number of outgoing edges. Not only two.
In the UML sequence diagram, the arrowhead has a clear meaning. Open arrowhead means asynchronous message, closed arrowhead means synchronous message.
I think PlantUML should, if possible, follow the standard presentation.