They've had a string of script-injection vulnerabilities with similar severity -- the last was less than a year ago:

https://confluence.atlassian.com/doc/confluence-security-adv...

Each time, they patch the particular vulnerable script, but they don't seem to have done anything systematic which would eliminate the entire class of vulnerabilities (like, say, adding taint-checking for unquoted, client-supplied content before running code as an OGNL script). So they keep coming up.

They’re widely used, which is not the same. If you’re a customer you’ll routinely encounter bugs, clumsy or inaccessible UI, annoying limitations, and performance issues which take ages to get fixed if ever. I would not assume that they’re better at security than everything else.