It's not something that's going to make it onto our stats page at least in its current incarnation.
I did a tiny bit of log scraping and aggregation to get some IPv6 stats. This is only the last 3 days worth of data, and had a few processing shortcuts that means it may not be completely complete. This is only looking at HTTP-01 and TLS-ALPN-01 challenges, and doesn't look at what protocol DNS was done with.
Currently approximately 17% of our validations are over IPv6.
The success rate for IPv6 is 15%. The success rate for IPv4 is 24%.
So at least in this somewhat simple sample, IPv6 has notably lower success rates.