Here's the harsh truth about the Castlevania series: no game in the series really kept the sanctity and purity of the original game, and the only Castlevania that can really be said to be super hard is Haunted Castle, which isn't a good game sadly. All the Castlevania games can be cheesed one way or another, be it Holy water boss stunt on CV1, alucard flying on III, etc.
But lets focus on the 4th gen games: Super Castlevania IV has already been discussed to death, how the whip is OP, how the rotating room wastes time, etc so lets move to the other ones:
Rondo, this one is sort of a modern fan favorite, but is also the easiest of the classicvanias when analized properly. First of all, it removed one of the key mechanics of the series: stair climbing. Rondo lets you jump on stairs, removing the risk factor of an enemy attacking you while you're climbing. Besides attacking, now you can also jump to evade. IV already let you drop off stairs, but not jump, Rondo basically makes the act of climbing stairs useless, it's faster to just jump to get across them.
Rondo got also rid of whip ranks. It may not seem like much but there's certain parts of the previous titles where, depending on the checkpoint, you would get stuck with a weaker whip until you find all the upgrades, also, it gives the player an option of an alternate difficult setting (doing a no whip upgraded run). Rondo just gives you one whip only.
Then there's the new sub-weapon, the Bible. Easily the best sub-weapon in the game, and once you get it on stage 3, there's no need to change it. It gives you a screen-filling offense-defense move, basically a giant spiral shield that will touch most of what's on the screen. It also doesn't consume too many hearts, plus the game is very generous with hearts anyway.
On top of that, there's the item crash mechanic. Honestly, with the Bible being as OP as it is, I find item crash a bit redundant, but the most importante thing about it isn't the attack part... (Cont.)