Rsync, how much risky to use it live?

rsync does not maintain file consistency like e.g. a database does; a file is copied as it is at the time rsync opens it, and as rsync can run for very long times, the file may have been changed since the time you started rsync. Whether this is "risky" is up to your assessment.

One of the ways to mitigate this risk is to run the same rsync command twice (or more) in a row, to keep track of any changes that were made since the start of the command. But if you absolutely need to maintain file consistency, you should mount the disk read-only for the duration of the rsync process.

Slightly different opinion to Jos... I'd say that if you need an active backup, use a filesystem that allows snapshots, like BTRFS. Its copy-on-write behaviour allows for instant snapshots of a system, which you can then create a remote backup with.

That still has the same problems as other active backup strategies though: if it isn't on the disk, it's not going to get backed up. Some applications buffer their work in memory before writing to disk (and need poking to write). Which of these really apply to you is debatable.

But there isn't anything inherently unsafe about copying files off a rw-mounted partition. Even one with a clunky old format.