To be honest, I don't think it has anything to do with the exponent part at all. Instead, I think it has to do with the fact that integers are inherently easier to grasp compared to fractions (which is exactly what rational numbers are).
For instance, it's much easier to say 2+3 = 5 than it is to say 1/2 + 1/4 = 3/4
So going back to the exponent example, it's easier to say
x^2*x^3 = x^(2+3) = x^5
than it is to say
x^(1/2)*x^(1/4) = x^(1/2+1/4) = x^(3/4)
So that's my opinion as to why rational exponents are more tricky to grasp compared to integer exponents. Of course, everyone learns math differently so maybe some find fractions easier than others.
