eg. amount of compromised WP installs (both total number, and percentage of WP installs (or course that are estimations, based on crawler data and so on, but still)), or number of known security bugs per 1000 sloc, or...
...
As I said repeatedly, I'm not trying to evangelize here, I'm just warning. I did that, done, period.
In case anyone has more arguments like "bugs are everywhere" and so on, I know that already.
If someone doesn't want to believe me, it's ok.
And you can't convince me that WP is secure, so no need to try. As said already, statistics, and I even found bugs myself (sql injections = beginner level problem, and it was found less than an hour after looking at a WP download the first time).