Are hand made WP themes safer than downloaded themes?

I usually build WP themes from scratch, and use the guides and queries WP uses; then I add jquery, css, etc., and I try to keep the plugins at minimal, coding myself what I need. well, plugins are packages of code after all. But external.
I think the weakness of downloaded or free themes relies in code that refers to external sources, or gaps where other code can be inserted (weakness to stop sql injections, etc); one person asked me about it and I replied as here; but the doubt haunts me.