Security of running a Docker container with SYS_ADMIN capability

I am trying to set up a Docker container with the UrBackup server. Because I use the Btrfs filesystem, I want to take advantage of its features by adding the --cap-add SYS_ADMIN feature, however, I’m wary of using it as I feel that I might be opening up to vulnerabilities by using this flag.

Practically, how much of a security risk is presented by using this capability?