However, you are leaving out the part where Labor has said they will end freedom of movement and the EU has said they will not agree to single market but no freedom of movement. So while the Labor position seems clear, it doesn't seem realistic and it is also unpopular with a large portion of the party (as would be any brexit position).
Antisemitism is also a major issue in Labor (not just Labor), although I get the strong sense that fewer UK voters care about that than one might hope. It still seems likely that some people who might otherwise be strong supporters of Corbyn do not enthusiasitically support him because of this.
Corby also seems to have a bit of an authoritarian streak, although it is hard for me to tell for sure since UK politics seem to be structurally more publicly heavy handed from the party leadership than US politics. It seems to affect how he responds to various issues and might make them have more negative effect than they might otherwise.
Another huge issue seems to be the raging battle between the human friendly side vs the business friendly side of Labor that should really be two different parties but can't because of the voting system (same issue in the US). I would agree that much of the vitrol against Corbyn seems to be due to him being on the human friendly side.