is it a standalone crypto that has not been launched on ethereum?
Don't you remember EOS?
EOS was just a piece of paper, no mainnet, no nothing, just a token and a piece of paper.
It gathered 5 billion USD in the ICO and when to top 10 in day 1.
Is EOS a scam? Doesn't look like to be a scam. Is it a good project? Not in my opinion.
What about Polkdot?
There are a lot of eyes in it a now. Certainly it is not the best time to get in. Is it a scam? Unlikely. Is it a good project? Unlikely as well.
And back to the question whether polkadot is a scam, it is not a scam, all crypto assets are volatile, they up and down, and sometimes, if their price go down significantly, they loose their position or coin market cap, this does not necessarily mean it's a scam.