I think the alliteration is what makes it a good insult fundamentally - it feels satisfying to shout in frustration: "Quit your obnoxious yapping, you absolute wankwaffle". I'd assume it also implies self-important ego-stroking nonsense? "Get to the point, life's too short for this wankwaffle!"
Can't explain what makes it a good insult if it isn't a good insult.
OK, I think I found a good example. Passing regulation that says "asylum seekers in abject poverty can't use their Home Office prepaid card to buy snowmobiles and timeshares" is what could be appropriately referred to as "wankwaffle".
As in: entirely pointless performative bullshit of no real-world consequence apart from giving some assholes with hate boners a bit of petty satisfaction.
It pales in comparison with the glorious cockwomble