Politics Knowers in the UK are about as useless as the Politics Knowers here, and the people who write about UK politics here are even dumber.
But, to sum it up as simply as one can do without making (valid) excuses: Brexit split the Labour coalition and didn't split the Tory coalition (aside from a few of their peacocking Nevertrump (NeverBrexit) equivalents), and there was never a perfect strategy for dealing with that. No love for Corbyn personally, also, though (excuses time) his treatment by the media has not exactly been even handed (even from the parts of the media that are supposed to be).
As for the basic Brexit question, the Politics Knowers always wanted it to be a clean battle between Brexit (Tories) and Remain (Labour), though I don't think that was actually possible and don't think a non-Corbyn Labour leader could've or would've pursued that either.
Throw in the collapse of Labour in Scotland, which was kind of Labour's fault, though not really Corbyn's...