Any data?, people online profess support to groups they don't to lend credence to criticism.
Within circles of known Labour voters I don't know any, this forum being hugely majoritivrly Conservatives is hardly a reliable weather gage for public opinion.
A large part of Corbyn's risk factor is implicit in that. You cannot win a general election on "known voters" of any party, because a very large number of voters aren't "known". They aren't generally members of any political party, don't engage in public debates, and are often too busy living actual lives to pay much attention to politics or politicians unless a major vote is imminent. I'm reasonably interested, but have noticed that discussing politics usually either leads to an argument, among those that are interested, or empties the room of those that aren't faster than half a dozen stink bombs.
"Known" Labour voters have at least one thing in common with "known" Tory voters - neither are likely to change their mind. Elections, on the other hand, are lost or won by who does the best job of attracting, or conversely frightening off, that critical block of floating voters that don't have party allegiances, and more than that, floating voters in marginal seats.
Over the years, my vote has gone to Labour, Conservative and once, yes, even UKIP. Sometimes, it's a positive vote for who I do want, sometimes it's a negative vote against someone I certainly don't want, and in both cases, it's informed by local conditions. And in the case of the UKIP vote, in EU elections, it was essentially a one-issue vote, which certainly is not the case in a general election.
If Corbyn/McDonnell are the labour team come the next General Election, my vote is very likely to go to whichever candidate is most likely to defeat a Labour candidate, because nothing any of the others stand for is likely to be less appealing to me, as a non-partisan floating voter with no Party allegiance, than the platform those two stand for. If that means gritting my teeth and voting for Osbornian Tories, or even Farage, so be it.
Personally, I feel absolutely no party allegiance. I vote on who stands for what, and what I think is in my best interests, which comes down to what I think is in the country's best interests. My belief is that that's why most floating voters float. And aren't "known" voters for any party. Even I don't always know who I'm going to vote for until I'm standing there, pencil in hand, looking at the ballot paper.