If you look up the documentation of the ETF, you'll notice that most (at least ALL those that I have checked) mention that there is a "counterparty risk", they are lending stocks. I've looked up Amundi for example, and all their physical tracker are lending stocks, 25 to 45%. It is a risk as if whoever they are lending the stock to bankrupt, they lose the stock. This can be bad, because there is also a liquidity risk. Some of the stocks they own aren't traded in large quantities, meaning if a lot of people are trying to sell, there is a problem, so in case of turmoil on the stockmarket you might not be able to sell as you'd like to, not only because of the direct liquidity risk but also because they're lending up to 50% of the stocks. And you'd better hope that whoever they're lending to isn't going down, or you lose an extra 50% or more.
Synthetic ETF are the same, but it's pretty much 100% that is exposed to counterparty risk, and liquidity is probable much worse than physical ETF.
I know that in here it's "get an ETF you dumbass", but to me it's really starting to be a concern when such a large part of the trading is done through them, and especialy only on an handful of indexes. Here's an illustration of why it's starting the be concerning :
View attachment 225339
Equal weight S&P 500 is increasingly outperforming S&P500, my
opinion is that it's because too many people are trading exactly on S&P500, which would result in overweighted stocks in the S&P500 to be overvalued.