I have found the summaries reasonably good, with 1 exception it got completely wrong, but I only ever want them to summarise a stack of notifications from the same app, and never a single notification. Unfortunately there is no setting for that currently, it's all or nothing.
Would be nice if they had an option only to summarise multiple notifications in a stack, and not to summarise once you expand them.
Especially since so often its summarising a message that is barely longer than the summary. It seems to sometimes decide not to do that, but still so often does.