There are actually a number of layout engine hints available for compact, minimize crossings, hard angles instead of curves, top/down vs left/right, and using hidden lines to force specific relationships.

It's scattered about a bit through the manual, but you can achieve most things you need.

Really, there's a Zen of PlantUML where you stop trying to control it and focus on defining the relationships.

Thx. I will take a look at those.

What I would like is way to specify start and end row and column number of nodes. That way it would be more predictable and changing the relations of two nodes does not cause everything to change.

In some diagrams you can use ->, -->, --->, -...-> to specify "ranks" between connections. The layout engine will space accordingly, depending on the top-down or left-right hint.

Yes, especially for statecharts, they get rendered in an ugly manner, totally different from how someone would manually lay them out.

Sequence diagrams come out all right, but I wish if there was some provision to lay diagrams out for automatic page breaks for a4 size paper, for example.