Import phpBB - incomplete due to user invalid email address (trailing space)

True. In import_scripts/phpbb3/importers/user_importer.rb around line 27 add .strip like this:

        email: row[:user_email].strip,

That’s not wrong, but since this code has existed almost a decade and it hasn’t been fixed yet, it’s also not clear that you’re right. It was easier to tell you how to fix it in the phpbb3 code rather than in base.rb (which I agree would be better).