Yeah, I use IK exclusively when animating my game characters, for both hands and feet. I have never found FK to be easier, and that’s in a game about swinging swords around! Human beings very rarely actually think about the movement of our arms and legs. Our cognitive power is devoted to positioning our hands and feet (and head), and we let the automatic functions of our brain handle where our arms and legs are going to go in response most of the time.
So when you go to animate something, like swinging a hammer or climbing a ladder or reloading a gun or taking a walk or sitting in a chair, what you’re thinking about is the movement of hands and feet for that. You don’t think about the position of your upper arm, you think about the trajectory of your hand. Trying to translate the mental process of positioning your hands into the computer process of rotating your arms is an extremely complicated endeavor.
If I could do my rig over again, what I would do is MORE IK. I would do a FABRIK implementation on the hands that ran all the way up through to the shoulders, FABRIK on the feet that ran all the way up to the hips, and FABRIK on the head that ran all the way down the spine. Just positioning 6 pieces (core-at-the-waist, left hand, right hand, left foot, right foot, and head, plus I suppose joint pits for the elbows) would make literally every animating task I’ve come up against way easier.
As for a giant spider, I wouldn’t worry too much about it having 50 bones. Even with dozens of them on screen, the performance cost of the bones is not going to be the bottleneck, especially since spiders have rigid exoskeletal legs and most of the vertices of the model will be single-bone weighted as a result… same actually goes for most of its body, too. Unlike mammals, spiders don’t deform a lot when they move because they don’t have skin over top of their skeletons, so the bulk of the vertex deformation will be single-bone weighted, which is super cheap performance wise. I mean, yes 50 bones is going to hit performance harder than 15, but even with a dozen spiders at once that’s what, 600 bones on screen? It’s just not that much in the year 2015 (unless you’re making a mobile game, I guess).
My game’s main character has NINETY-FOUR bones with vertices weighted to them (I know, right?) plus probably 90 more that don’t. I can slap 6 of them in my scene (not just the skeleton, but the entire character BP, with the full running anim graph) and I still get 60FPS.
The real cost to you in spider design (assuming you’re going for realism, and it is my opinion that anyone who would use 50 bones to rig a spider is going for realism since they’re doing a minimum of 5 joints per leg rather than the standard 2-3) is going to be the material cost of fur rendering since spiders are all covered with fine hair. THAT’S where your bottleneck is going to be on something like that, and honestly that’s the bottleneck on most games these days: materials. Shader complexity and draw calls is what chews performance up. We’re sort of past the point of sweating triangles and bone counts where modern graphics cards are, at least IMO.