Personally, I don't think the castle looks all that inaccurate. It might not look exactly like it did in the original, but it's still pretty close, anyway. I don't think it's necessarily anything to rage over.
Anyway, as for my opinion on these 8-bit remakes, I think they're extraordinarily good. They play exactly like the NES games (4-6, in particular), and are very fun to play. However, I do find these remakes to be a lot more difficult than the originals. Mega Man 8, especially, since I could hardly get past Frost Man's stage. Also, people could say what they want about the "Jump, jump! Slide, slide!" warnings, but I actually found them to be pretty helpful. Since the Famicom remake lacks those, it's a bit harder, but maybe that's just me. Also, I'm glad to see that they've implemented the shop in 8. That's one thing about the Mega Man 7 remake that bugged me a little. Lastly, I hope 8-bit Mega Man 8 will be customizable, just like 7 was. That way, it'll be possible to make things like this: