Security is very subjective: Some security features are a serious threats to one's (computer) security, while other protect people. For instance nonfree boot software that you cannot change (and is nonfree) is often considered as a security feature by device vendors while free software users consider that as a threat instead. In contrast, many mostly invisible security features protect users like ALSR, stack protections, etc, with almost no cost for (practical) freedom.

The freedom-privacy-security-issues page has an introduction to various issues affecting smartphones and tablets in general.