IMO, it is frequently seen as a solution in search of a problem, that breaks things and makes things harder when used to no real benefit. Whether any of that is accurate is left as an exercise for the reader.
Importantly it really did not focus on actually supporting a migration (ie, IPv4 only nodes connecting to IPv6 only or visa versa). And yeah, it scrambled up lots of things for very little gain - even big players like Google took a while to support it across their entire stack.