Specifically - what is the opposite?
And since you found the 'opposite to be true' - does it follow from that that you found something which was easier? If so, what then?
What does efficient mean? In practical terms?
ES/SPY moves substantially on a daily basis, yes?
Does not a wider spread make it even more difficult for the amateur? In practical terms - how is it 'easier' for an amateur to 'exploit' an illiquid stock?
When I was just starting out I found out after the fact that I had just entered a listed, but very illiquid stock that had days where it barely transacted. I was just lucky to get out.
It's possible. I wouldn't know.
Day trading - I only ever traded the ES seriously. I dabbled in CL for a few months - a market I assume should be considered less efficient than ES, but found it to be too wild and unpredictable to me.
ES is a technical market and predictable to a certain degree. Just observe where the
morning HOD was printed and later on the current HOD. These are not random levels. Just from my general statistics I know with a 90 % certainty that ES put in the HOD already today.