It would help to list examples of what these "tighter" combos are because unless you're playing Angel in 02, most combos are no different from SF bnbs. Spacing isn't necessarily "more complex" but there are more options available because of things such as running, rolls, hops, etc. But it boils down to the rock/paper/scissors kind of game during the inside-game. Standing attacks against hops, hops against sweeps and other far reaching grounded normals, and sweeps against standing attacks. There is a few more layers and nuances such as Kensou and Athena being zoning characters, it's not a free ride to try to hop over their projectiles so the player has to play more traditional SF footsies to get around them such as landing a good full jump, or just rolling at the right moment. Since movement is so liberal though, the player could just simply be in range or be outside of range of something an opponent does and just get a full punish off of it.
In Street Fighter though, there is certainly less subsystems especially if one counts ST and A2. But, due to limitations of movement and available universal systems, the player has to play a stricter ground game, really bait out opponents to do a bad jump-in, play footsies, and really abuse spacial advantage. Such as, a player has to go through a series of set ups just to make Dhalsim to do a bad drill to only punish it with a far st.MP or something minute as that. So there is this type of discipline that comes with SF, especially high level SF, that is just as hard as trying to learn KOF. The difficulty doesn't necessarily lie in the amount of things a player needs to learn, not that it's hard to learn KOF combos outside a few characters or learn what the hell a hop or roll is, but in the mental demand it makes of the player.
Seriously, if you're a top level player at SF (not intermediate, top) and really know how to space with normals, KOF should be a breeze to learn. Vice versa. If you're going into KOF from SF and you're really good at SF, all you really need to know is that rolls are the new ST jumps (as in something that is a huge committal that goes around your attacks but you could earn a large punish off of it/high risk high reward), hops are a form of Standing Jab/Strong/Fierce that covers essentially the same space but allows a player to move forward or backwards while doing it (that is doing an early hop attack, which is susceptible to whiff and the opponent could let it whiff and punish it by crouching underneath. If the hop attack is done a bit later, it acts more like a jump-in but is more susceptible to anti-air before the attack actually comes out), and that there is no medium punch or medium kick buttons.
It works vice versa for KOF. When going from KOF to SF, one has to recognize that there are essentially no hops, rolls, or running (some may have dashes though) unless you're playing 3s for whatever reason (universal overheads) so all a player could do is commit to jumping but one has to earn that right to do so. Since there are two more attack buttons and that grounded specials such as projectiles have a bit more dominance on the ground, the KOF player will have to recognize that he'd have to (generally) be in enough range to counter poke the higher start up attack that he wouldn't be able to poke against once it's out (projectiles) and be ready to respond against a bad jump. So essentially, while weaving in and out of normal attack ranges and maneuvering to limit or get over projectiles, he's playing footsies. Then once the player can earn the right to put the opponent in the corner, the player has to maximize on spacing and get the opponent into a fireball trap and maximize the situation so that even if the fireball misses because of a jump or something, the player could anti-air the opponent.
How I perceive KOF and SF, they're merely extensions of each other but with slight shifts in design here and there. At the core, they're really the same game. It's just that it's easier to reach higher levels of KOF more quickly than it does with old school SF. Especially if you just want to tier whore in KOF, in 98um just choose Krauser/O. Geese/Eiji, Kyo/Iori/Daimon in 98, K'/King/Kasumi in 02um, Kim/Athena/Kula in 02, Kula/Gato/Oswald in XI, K'/Raiden/Shen Woo in XIII arcade. It's not a free ride, but the player could really just play their own game when using teams like these and ride pretty easy. You could choose O.Sagat in ST, but you can't really just throw dumb projectiles all day (has to be smart ones) and really know the match ups and play those match ups. Even in 3s, when you pick up Chun Li you have to learn each hit confirm (pretty easy) but then go learn stuff like SGGK with her and learn to confirm cl.RH and sjc into SA2 among other things.
So really, I've stated why I'd say KOF is not harder to learn as SF. It might be just as hard, but I really doubt at the higher levels it's more difficult than ST or so. I'm willing to be stated wrong, but you have to prove it to me first with actual content if you're going to stand by your stance. What's harder and more complex about the spacing? Is it Chris that keeps doing hyper hop j.CD that locks the player down, then the player uses neutral hop j.B or D to poke at the right spacing to punish Chris, then encourage Chris to start doing super jump j.CD in which the player uses this opportunity to run underneath and anti-air with a cr.B into a full combo, then Chris starts to mix-up his slides and spacing in which the player could respond properly with hop-ins or what not? Is grappler okizeme that complex? Is learning how to guard roll against bad 2-in-1s that complex? Is anti-airing a hop with st.A that complex? Is it hard to recognize when to alternate guard against a bad tick throw attempt? There might be a lot of new things that intermediate SF players going into KOF that they need to learn, but by no means makes it hard to learn nor would it take much time to get caught up on what's going on.
Maybe if you're utterly bad at fighting games, it might be harder to learn KOF than SF because you get steamrolled by rushdown characters able to get in full combo each time they hop or something because you're bad at blocking and using OSs.