Can't vote for Sanders; he's a crazy old radical socialist from the 1960's who believes in the Lost Tribe of politics, and he'll never get his crazy universal healthcare through Congress and he praises Cuba and Stalin, and he's old and had a heart attack and he's not even a real Democrat, and his Bernie Bros are just like Donald Trump and, and, and...
Oh, look Biden made another embarrassing gaffe!
I think the difference between BernieBros and BidenBoosters is how they often identify with the problems with their particular candidates.
Biden would not have been my first pick as a candidate, but I think he is best of what's left. However, I do recognize his track record of making embarrassing gaffes is a problem and it could cost the democrats at least some votes. (I suspect the majority of BidenBoosters here feel the same way).
On the other hand, when you point out the problems with Bernie, you often get a complete failure to acknowledge the problems by the BernieBros. Point out how his statements on Castro might cost votes (especially in Florida), and you get "Obama said the same thing/its true/etc.", when the proper response would have been "Yes it was a mistake." Point out how its foolish for Sanders to attack "Democratic elites" since he will need their support in the general election, and you get "Well, they had it coming". Point out how Sander's self-labeling as a "socialist" could be problematic, and you get "Well Republicans will attack anyone". Point out how the vast majority of people are against banning private health insurance, and you get some sort of claim about how "If you don't support BernieCare you want to see people die in the streets, besides look how great of a system it is!".
How dare you do Trump's work for him, passing on all the disinformation of the Russian bots and Putin must love you, and there is nothing wrong with Biden, he has always been this way and how can you even when the point is to get someone anyone elected just shut up shut up already!
I think the issue is that BernieBros are not just dealing with the gaffes themselves (which might be fair game), but they are trying to suggest that it is due to senility/dementia (which is at least partly speculative).
And keep in mind that at least one poster who has been supportive of Sanders (Sideroxylon) appears to have accepted a doctored video of Biden as valid.