I think the craziest example of Bernie failing to attack Biden is: Biden literally "stole valor" from the Civil Rights movement, pretending to have been a desegregation activist who did sit-ins. Bernie actually got arrested as part of that movement. But never called Joe out!

Bernie is reluctant to "toot his own horn" about this stuff, rightly so. But Biden insulted those who put their bodies on the line for that movement, and Bernie could have spoken up for his friends' honor. Especially insulting when Joe Biden was best friends with segregationists.