Are there bad investments in terms of ETF? How would you quantify a bad investment?

Criteria for bad ETFs are

#1 of course ETFs, that have low returns including dividends over several years.

#2 ETFs with:

low assets under management (AUM), i.e. roughly less than 100-200 mio USD

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high fees (total expense ratio = TER), i.e. higher than 0.30% per year, though some

technology ETFs with fees up to 0.80 % still can be very successful

.

active managment (because stock picking mostly does not work, at least outperformance of an index is an illusion) ... empirical exception currently, and no advertizing here: the ETFs of ARK company: ARKG, ARKK, ARKW ...; they are actively managed and still very successful, but maybe the time since inception is to short to be able to decide on ARK success

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commodities ETFs

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bond ETFs (traditional investment but future value/success doubtful)

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inverse ETFs

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leveraged ETFs

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ETNs ('exchange traded notes', counterparty default risk)​​​

Yes, there are bad ETFs too. Some ETF have extremely high expense ratio, tiny AUM, poor performance, high tracking error, etc. I will avoid these type of ETF.

Not all ETF are created equally, some ETF do not track an index. Or some ETF might be actively manage and have expense ratio of 1-2%. Some ETF might be inverse or leverage type.

Please read through the fact sheet before investing in any ETF.