Do you have a real-world example?

Joins haven't been a problem for me, mainly because this approach doesn't constrain you to a tables-and-joins model of the world. With Prevayler, for example, you treat things as a big object graph, so there are no splits to join back up.

Of course, it could be that some problem is just computationally intense, but I can think of a number of approaches to lessen the impact of that in a NoDB system.