It is a scam because it never delivered on any of its promises.
Ethereum was supposed to be decentralized, but it wasn't.
It was supposed to be secure, but it wasn't. The DAO mess and lots of other vulnerabilities that were never fixed.
It was supposed to be fast and cheap, but it wasn't. Vitalik was advertising it as "fee will never rise" but we all know how that ended up.
It was supposed to have an immutable blockchain but it didn't. The roll back killed it.
and so on...
When they advertise something but sell you something else, that is the definition of a scam.
The DAO hack and solutions from Vitalik Buterin after that, shown Ethereum is a Centralized Project, not Decentralized like how he promised.
Years after the DAO hack, Vitalik managed to make success for Ethereum and some times there were hypes like Flippening against Bitcoin. It's another hilarious piece in Ethereum history.
From infinite supply, their team tried to use Burn mechanism to make it deflationary but still with infinite total supply. It gone well for several months before the Dencun upgrade that lit up a lot of hope in Ethereum community for cheaper transaction fees. Unfortunately they forgot a fact that efficiency of Ethereum Burn Program is purely depended on how expensive users pay for on chain transaction fees.
After the Dencun upgrade, with cheaper fees, their burn program failed, and you can see everything from failures of Burn program as well as unrealistic Flippening speculative hype.
https://ultrasound.money/