I looked into this from the French point of view. Looking specifically at Lyxor and iShares. The best tax-advantaged account in France is a called a PEA, and only EU stocks are eligible.
The S&P500 tracker ETFs that are eligible are therefore synthetic. I started digging into what that actually meant. They don't hold any S&P 500 stocks, and reproduce that index through swaps. They only have a single counterparty listed in the prospectus. I asked some friends in the business if anyone insured against counterparty risk. For Lyxor, the counterparty is SocGen. Lyxor is a subsidiary of SG. So if SG goes bad, and Lyxor is solid, but Lyxor has no insurance, is there a problem? Or is it left holding what amounts to shitty SG paper?
Inevitably, it's complicated. Lyxor holds a basket of European stocks (Daimler, BASF, Porsche, etc. google this file name and look at p.30 for an example: AR_2003908_FR_20150331_FRA.pdf) and does a swap with SG for the S&P 500 performance. But no, there's no insurance. So if SG goes completely bust, but Lyxor is somehow fine, it seems SG won't make good on the swap, and your S&P 500 growth fund will actually be a bunch of German industrial companies. Took a couple of hours of digging to find out even that basic info.
Maybe I'm just ignorant, but it seemed odd to me!