Well, I don't doubt what you say, but it wasn't my experience. True, the colour is not really black, more gunmetal grey, though it does seem darker inside than when I painted outside. But it does have good covering power, it hid a previous coat of jet black after the second spraying. And a week later, no sign of flaking.
I wonder, did you adhere strictly to the instructions, particularly the drying times, ie 30 mins after the first (primer) coat then only 5 mins between subsequent coats? I've noticed some Hammerite paints are particularly fussy about this, so maybe that's something to do with it?