Cross is, um, kinda tough ;-)
I've discovered that to race cross and have fun you have to... show up.
To hit the middle third of the pack takes some technical skills, some practice on the cross specific stuff like dismounting and remounting, runups, sand and barriers... and Cat 4 road fitness.
To hit the top third of the pack appears to take Cat 3 road fitness. And a lot more of the technical skills. And this is in C class.
But all is well... it doesn't matter how hard it is, it matters that its consistently hard so you can benchmark your progress.
That said, I do think USCF should consider forcing a rider to race cross at either their road cat or mountain biking cat as a minimum. If they lack the technical skills, then they can be a backmarker in the A or B class while they learn... just like the beginners are a backmarkers in the C class while they learn and make plans to be more fit next year ;-) But at least they (the fast road and mtb riders) have the fitness. You can learn skills in a season or less if you have the fitness... but cat 3 fitness might take 2 or 3 years depending on your ability, age, and what you are starting from.
Just a thought.
Oh, and it's all irrelevant because racing cross is about having fun, suffering miserably, and getting faster for "next time".