All ETFs have an expense ratio, but that is not hidden. What is more insidious in a brokerage account is what happens to your cash. Are you getting a good interest rate on the cash that sits around in the account? I imagine that zecco won't let you buy partial shares of an ETF, so any cash you have leftover just sits there, right? Any dividends paid by the ETF have to earn interest or be reinvested right away as well. So cash can be a drag on your expense minimization after a fashion.
With a typical mutual fund you get free dividend reinvestment if that is what you want and you can buy any dollar amount and get fractional shares. Therefore, you can avoid having idle cash lying around.
That written, I use ETFs with my WellsFargo account that gives free trades and very good cash sweep interest rate.