I like em
But im interested in what all the concern is about their use
DHH isn’t (or wasn’t?) a big fan of engines: http://weblog.rubyonrails.com/2005/11/11/why-engines-and-components-are-not-evil-but-distracting/
Also the core team doesn’t seem to worry much about breaking them since they use non-public Rails API methods.
To the OP I think the 3rd option is your best bet. The functionality you describe is easily categorized into separate web applications.