Or for advanced situations "selectively efficient" - using something other than Electron for an app that is intended to be cross-platform because [insert-alternative-here] is smaller/faster/other may be an example of premature optimisation and using something else (especially where that involves learning something else) could mean trading off elsewhere.
Not just Electron but Sciter too in that sense.
"electron can have a small fingerprint if the dev team is carefull/good enough"
Only to some extent. You will have two separate process (at least) in any case and so two sets of the same system libraries loaded in them, IPC between them, etc.
Main task of browser engine is to provide safe browsing experience. Presenting HTML is only second its task and so browser based UI will always be sub-optimal (at best).
That said, electron can have a small fingerprint if the dev team is carefull/good enough, and is hte best option when your app have to be able to read HTML (and copy HTML formatted string). To me, an email client is a good way to use electron for an app.