OTOH it's possible, even likely, that the manufacturers recommend against gooseneck adapters purely as a CYA. Personally I feel that the builders realize that they are making a marginal frame in the pin box area and allowing someone to use an adapter creates an unnecessary risk to their marginal frame/pin box area.
Personally I've read of quite a few people that have used gooseneck adapters successfully versus 1, maybe 2, that have had an issue.
I think they create unnecessary stress in the pinbox area, but I don't feel like it's a personal attack if someone wants to use one.