The most famous "spoiler" was Perot, not Nader. Bush v. Gore was an extremely tight election, but there were third parties that drew more votes from Bush than Gore, and wer within the margin of error, possibly counterbalancing Nader's effect. Whereas with Perot, he received 19% of the popular vote, most of which were clearly drawn from George Bush, ensuring a Clinton win.
"While many disaffected conservatives may have voted for Ross Perot to protest Bush's tax increase, further examination of the Perot vote in the Election Night exit polls not only showed that Perot siphoned votes nearly equally among Bush and Clinton, but roughly two-thirds of those voters who cited Bush's broken 'No New Taxes' pledge as 'very important' (25%) voted for Bill Clinton."
[1] https://en.wikipedia.org/wiki/United_States_presidential_ele...
How can you blame Stein for a few thousand lost votes when millions of other voters who voted Democratic in the last election stayed home because they didn't like your candidate? Do you really think those thousands of people that didn't stay home because they didn't like your candidate, but went to vote, and voted for Stein, would've voted your candidate instead? No, best case scenario, they would've stayed home just like the others, otherwise.
Also, in some states tens of thousands (more than those that voted for Stein) actually did go to vote, voted on local elections, and didn't vote for president. Either way, blaming Stein is just laughable.
If that margin is applied to each state (which isn't quite right but whatever), that's roughly enough to flip Michigan but probably not any other states.
So, true that you can't pin the blame on Stein, but she did have an effect, partly measured by the vote, and partly unmeasurable in terms of enthusiasm dampening, etc.
Same sort of thing with Nader - you can't quite draw a mathematical line that he flipped Florida, but all his Gush/Bore stuff - who knows what affect that had in terms of people not voting.