A couple of thoughts on branding and how it relates to what stories hurt which politicians and which ones don’t.
For the conservatives, their brand has for a long time been “we are the sensible, serious ones who can actually get things done because we are competent.”

Historically they have been vulnerable to having their incompetence and sleaze exposed (see: John Major).
Johnson, however, is branded himself as a ‘lovable oaf’. This is a clever tactic as it means he is basically immune to stories about his incompetence.

They don’t hurt him so much as confirm his brand and public image. Boris did something ridiculous and stupid isn’t a big news story because, well, he’s always doing that. It’s his brand. With Johnson at the helm the Tories get to have it both ways.

The party can pretend they’re great and competent while any evidence to the contrary is just Boris being Boris.
For Labour, (and the left), the brand is, well, kindness and solidarity. Corbyn especially, who has a history of fighting for the cause, nails down the “brand identity”

This is Labours strength. But also its weakness. If you want to portray yourself as kind and keeping solidarity, all that needs to happen is to find someone in your party who has done something cruel or racist.

Claims of racism against the Labour Party are damaging because it’s damaging the party identity and ideology. Claims of racism against the Tory party not so much because they a) don’t define themselves in opposition to racism and b) nobody is surprised when a Tory is racist.

In fact, it's more surprising when a tory isn't racist or homophobic. It goes against the brand identity.