exactly this: min xx USD at CT vs. <CHF1 at IB for each trade. this gives you a strong incentitive to have much fewer ETFs with CT than with IB and to convert your CHF less offten into ETFs, hence having more unproductive cash lying around. I, for example, distribute my savings on 6 ETFs each month, which costs me about 2$ vs 150 
and another thing: CT (at least half a year ago) could not apply the source tax treaty btween CH and USA which means you directly lose 15% of dividens from US.
ah and some people here found that CT has worse forex rates, about 0.5% compared to IB.