At Netflix, we found in quite a few networks IPv6 had much better QoE than IPv4. In a limited number of cases, IPv6 was very broken. It was almost always equal or better.
Much smaller POV, but when I was dealing with ISP things in past I've found that on many networks v4 might have a nice, redundant blend while frequently v6 is some mix of HE+something or singlehomed HE, which disappoints me. But then the fact that their ports are basically unused other than for the usual G/YT/FB traffic makes the v6 pretty uncontended.
Luckily we have embedded OCAs and/or peering with almost every network of significant size, so their choice of transit rarely comes into the mix.
or when HE has bad/broken routes...
Did you have a sense of whether QoE diverged due to differences in the networking equipment or differences in the protocol itself?
There's an awful lot of very old IPv4 equipment still hanging around, but most of the IPv6 stuff is newer - so if IPv6 is working it's probably a more recent piece of hardware.
I believe in 1 case it was pathological issues with some networking equipment, but this was a recent thing, so I haven’t looked into everything yet.
Does who (ISP and end user) enabled IPv6 tend to have good network equipment / end device?