Roundcube doesn't support Gmail's oAuth over IMAP setup so that is why it is viewed as a "less secure app". The basic complain here is that your gmail password has to go though Roundcube before get to Gmail. Roundcube still supports all of modern encryption so your password is encrypted between your server and Google's. As far as I know Gmail's oAuth over IMAP setup isn't part of the IMAP specification, but it could probably be addressed with a Roundcube plugin.