Your experience grading tests for ETS will depend a great deal on the laws and practices of the state board of education in charge of the exam. I've graded for several states through them, and most states are a fine, if not a delight, but Texas for some reason has the most obtuse rubrics and the meanest proctors out there.
If you sign on with his company, gird your loins if they ever ask you to grade for Texas. The states that have the cleanest rubrics break it down to digestible chunks so that you are never in doubt as to whether or not you are in agreement with the rubric. Not Texas. Their rubrics are the purplest prose you have ever seen in your life.
And they wonder why graders have such problems with it? Have they read it themselves and tried to decipher it?