I've never looked at the full ISS ratings, just their publicized top ones. But generally speaking, I would say all the ratings you find will be hit or miss. That's scouting. You see it with the draft records of NHL teams, you see it with posts here, and the same goes for all the draft publications. They have guys they're high on which sometimes end up looking like bombs, sometimes like brilliant picks. And vice versa. I'd say the best thing is just to read as many different sources as possible and factor them all in. And compare them to your own observations on any prospects you've seen yourself to get a better idea of which agree with your own style best.
Personally, I think that (as a sweeping generalization), Central Scouting tends to rank with more of an eye to NHL potential, a more traditionalist approach of scouting, where size, physical ability, etc sometimes trump pure skill or intangible instincts. Whereas Redline is more willing to give props to the smaller skilled player, and take flyers on some guys whose whole seems to be greater than the sum of their physical parts. Again, sweeping generalization, but that sort of approach tends to make Redline more "fun" to read, and maybe better at indicating "hockey talent", which is not necessarily the same as indicating "NHL potential".
I generally find that I like The Hockey News best for a quick and shallow overview of the top-60ish, probably because they get as much from their ear-to-the-NHL-scouting-ground as from any observations they make themselves, whereas the other publications try to go it alone on their own observations. But for more detailed info... a blend of them all spiced with your own preferences and observations is ideal.