Hello,
I have made 4 trials for iStock Illustrator Application Sample Upload , and every time i got rejection , with the same reason :
We are sorry but this artwork is not what we are currently looking for in terms of composition, color usage, use of line and shape, concept, detail and/or file complexity.

However I am not expert in illustrator , but the same illustrations sells good on SS and FT.

Is it that hard to be accepted as illustrator with iStock

Yes, their standards are higher....at this particular moment in time.

Yes, and once you have passed their "exam" they accept any crap you can upload�