Fun Fact: You don't need to be an expert to beat Super Castlevania IV. Just power up your whip, get the boomerang, triple-shot it up, and then you're neigh invincible. Even if you get hit, enemies deal crap for damage. The only real worry comes from elements of instant death.
Super Castlevania IV is sort of crappy in terms of challenge and polish. The level design very rarely pushes you to do anything difficult, enemies have needlessly high amounts of health while lacking the cunning to make them engaging, some bosses have unavoidable attacks or are easily trivialized, the whip-swinging mechanic is under utilized, the whip's range and aiming ability obsolete almost all of the sub-weapons and any semblence of strategy, the game has a raging axe armor fetish (And bat fetish for the second playthrough of the game), and the only enemies that are exploited effectively are the bats (occasionally), floating eyeballs and skeletons.
However, Super Castlevania IV is considered good because while it's level design isn't exactly creative from a challenge perspective, it combines colorful, detailed visuals with music to create a sense of immersion rivaled only by Symphony of the Night in the entire series. While it never asks for much in terms of challenge, it also has enough variation in the level design to not feel bland either. It's also got very flexible controls and mechanics, which makes it a very approachable game.
I don't feel it's enough to get me to like the game a lot, but it's unfrustrating and immersive, which for a lot of people, is agreeable.