... This structure may never cause problems, but I think there is potential for problems in fast-moving markets. This function is really a sort arbitrage. Arbitrage can be hazardous when things are moving fast...
This mechanism is indeed an arbitrage, and it is intentional. The purpose is to keep the ETF price fixed to the price of its component stocks.
I will admit that I have not read prospectus of the recent ETFs, but the earlier SPDR's prospectus said that
anyone could assemble a collection of individual stocks in the right mix, and deliver to the trustee in exchange for the ETF shares. And conversely,
anyone could buy the ETF shares, turn them in to the trustee and demand a conversion into individual shares to be delivered back.
While I would not be able to keep track of the price of my ETFs in real time to see if it tracks the individual stocks, I would think many brokerages would use computers to detect any mispricing between the ETFs and individual stocks, and immediately do a simultaneous buy/sell as appropriately to arbitrage away any mispricing.
The above would ensure that I get fair value when I hold these ETFs. I would not get any premium, but not suffer from any discount either. And this is all good.
I would prefer to see the fund manager handling this function, just as the fund manager handles it in traditional mutual funds.
The ETFs can be traded in the open market, and not going through the MF manager. So, I do not know how an ETF and MF can be handled similarly.
My discussion so far has been about SPDRs or ETFs that are passive indexed funds. The constituents are defined and declared in advance for anyone to do fair pricing. When I held some of these early SPDRs, I even had the voting rights to the constituent shares.
Back then, some sector SPDRs had only 20 or so companies in them. A telecommunication sector ETF may define 100 shares of the ETF as consisting of 20 shares of ATT, 15 shares of Verizon, 10 shares of T-Mobile, etc... And because of the legal structure, you had to buy these ETFs in round lots of 100 only. Else, you would have fractional ownership of the shares of the constituent companies.
The above SPDRs have no manager, only a sponsor and a trustee.
Anyway, they have now actively managed ETFs, whose composition is subject to change by their manager. However, they are transparent, meaning that the composition is updated daily, in contrast with MFs that only disclose their holdings periodically.
Can shenanigans been done with these actively managed ETFs? I don't know.
PS. I just now recall that the old ETFs that could be traded only in round lots of 100 shares were called HLDR ("holder"). These HLDRs are grantor trusts, distribute dividends directly to shareholders, who also retain the voting right of the shares. These HLDRs may be too cumbersome to maintain, and they do not exist anymore.