Sadly, in my experience, ipv4 is generally more reliable than ipv6 still.
Set up two hosts, host A and host B in two different data centers. Make them send HTTP requests to each other over ipv4 and over ipv6. You'll see that latency spikes, packet loss is more frequent over ipv6.
We’ve observed this in end-user devices, especially on some ISPs.
It makes sense if the overall adoption and resource allocation are comparatively smaller, making individual or small-group coincident spikes more impactful against the amortized whole.
It’s a lot like a market with low volume/liquidity. Someone wanders in with a big transaction and blows everything up.
