I have moved outside my main subject, which to be honest is not generally on the parental radar until A-level and then they don't understand it. So, for 10 years I have had a relatively complaint-free existence (ish!).
Now, I am teaching maths to year 7. Woh! Lots of complaints. But they are all that it is too easy. I would get that, but they are up to speed with other classes and the curriculum. It's not like I am teaching times tables while the others are doing calculus.
(Although in my mind, I would take a complaint of too easy over too hard - based on many years experience of students going into block mode in science when they don't understand something. Going too fast is much more damaging than too slow and differentiation is an art at times!)
My friend who works in management says that maths and english teachers are the ones who get it the most. And maths was the subject that she had to mediate the most.