Here's the way I see it:
Hillary Clinton's email server scandal had roots in some very real and legitimate questions about her judgement. Even if you accepted that there was nothing criminal to her email server, or at least no ill-intent, it looked really bad, and was definitely a stupendously dumb idea. Those questions allowed darker insinuations to flourish. It was also legitimately being investigated by the FBI to ensure that the emails contained on it were not classified and to determine if there were any crimes committed. All of that hurt her with voters, a lot. She would likely have been president without it. A lot of people voted for the Devil they didn't know (Trump), or Johnson, or just stayed home.
There was a lot of mud thrown, and a lot of disinformation about it, but the reason it hurt her was that at its core, it raised legitimate questions about her judgement regarding the email server, even if everything was technically legal or well-intentioned.
This Burisma scandal, in comparison, is comically transparent and all the information about it is ultimately being sourced from within the Trump campaign. It's the insinuations about Hillary's emails being for nefarious purposes to do human trafficking and such, but without the verifiable core story that calls into question Biden's competence or integrity. That's the missing piece, and that's why this attack line simply won't work without real actual evidence that didn't come from a member of the Trump campaign.
And that's the other issue. The Trump campaign has been so dishonest the last four years that nobody believes them about anything anymore. They could have a legitimate smoking gun, but people are largely conditioned to ignore the crap he and his acolytes say. And that much is on nobody other than Trump. If they want the Biden/Burisma stuff to stick, they need an independent entity to bring the evidence forward. Not Rudy Giuliani leaking it to a biased news source that did none of their own investigation.
e.