I'm sure there are big effects from obvious things like amount of alcohol consumed, individual susceptibility, hydration, and so on.
But holding all that constant, I've also seen some unsupported claims that different drinks can cause varying severity of hangovers, due to different chemical composition - additives (e.g. sulfites) or other naturally produced organic compounds besides the ethanol.
Easily found mass-media articles (e.g. this one and this one) pretty much exclusively talk about congeners. It's unclear whether that's just because that's the one thing that's been established in studies or because it's the dominant factor, and it's unclear how strong the effects are.
Is there research to either suggest that congeners are the dominant factor (not just one of many), or that there are other factors? And how strong is the variation in severity, whatever the causes may be?