I've not used them. Shouldn't be a big security risk related to outdated themes. However, there are lots of security risks related to outdated versions of WordPress. The consideration is whether your theme will continue to work, even as WordPress pushes out updates. Most themes seem to transfer fine to new WP versions, but this could vary depending upon the theme and its features.
How's that for mud in the water? But truly, the answer is . . . it depends.