Yes, I've heard that the knockouts are often a problem. If your sparky wants an easier life, he should agree to fit a plastic CU this month; he can't have it both ways!
I don't think the earthing issue is anything to do with the new CU but, presumably, your current earthing may not be up to standard. I gather that the new metal CUs are themselves part of the earth and the individual circuits are thus connected directly to the casing, which slightly simplifies matters.