This is not CF WAF's first rodeo https://news.ycombinator.com/item?id=20421538
Cementing its track record as a product that mostly doesn't do anything except for occasionally break the internet here and there to keep things fun and interesting.
I'm not sure why "WAF has false positives" makes it useless, nor would I say this is anywhere near the scale of "breaking the internet" and I'm not even fan of the concept of WAFs in general.
we've used it to rescue some vintage appliances that are basically unsecurable.
> a product that mostly doesn't do anything except for occasionally break the internet
I wouldn't say that. The postmortem you referred to links to another CloudFlare blog post - one about a pretty serious RCE vuln in Microsoft SharePoint that was blocked by their WAF: https://blog.cloudflare.com/stopping-cve-2019-0604/