I think it depends on the application and how heavy its tabs are:
Browsers use threads/processes per tab because each tab is often as heavy as a whole application by itself. Additionally there are security and isolation concerns which are resolved or mitigated by running each tab in its own process.
Dolphin uses one process per window because each tab only adds a minimal amount of overhead compared to the whole application, so making each tab its own process only adds overhead without any benefit.

Konsole's shared process model is perhaps a little outdated as that model was intended for memory-constrained environments to add features without increasing the memory use (significantly).