To name one: You realize after finishing your code block that the first two lines of it actually shouldn't have been part of the code block, but should have been normal text above.
Go ahead and try to make this change with the new system.
Compare that to what you would have done with the old, plain-text system.
Oh, and you can't easily get the underscores back -- you'll need to retype the variable name.
Slack can't seriously expect us to apply clumsy workarounds to their single most important feature (text-entry) that many of us use hundreds of times per day.
In particular since the solution is as trivial as adding an off-switch.
(Or I guess Cmd+Shift+V)
* move the cursor to 0,0
* hold shift and hit down-arrow twice to highlight the first two lines
* continue holding shift and hit left-arrow once to deselect the EOL on the second line
* use the keyboard shortcut for toggling code blocks (cmd-alt-shift-c)
EDIT: formatting