Rick--
If they "they never really felt like butter they just looked really burnt", then something's not right; I'm not sure I would say it's timing. It is hard for me to put 'time' (as in X minutes or Y hours) in the same context as 'barbecue' even though I know we all do it as it can make writing about what we do easier; it's just that it seems to put the primary emphasis on the wrong variable. I'm not sure I'm making sense here.
Yes, it is quite possible to overcook ribs without having them fall off the bone, especially since you didn't use foil. For me. ribs should have a firm, almost crisp, surface-feel (from a combo of the bark and the somewhat drier meat at the surface); the meat underneath (all the rest) should be tender, soft--but not mushy, and should release from the bone with a bit of a tug (I'm not a devotee of fall-off-the-bone).
The meat thickness (before cooking) should be proportional to bone size (for best results, imo) so your feeling that your spares were huge but had thin meat might be the primary problem especially if the fat-lean ratio was off with the meat (thin as it was) not having enough fat interspersed. No matter what size the racks, no matter what the fat-lean ratio, the ribs (or brisket, butt, picnic, chuck--this applies to virtually any meat) will be done 'correctly' at some point. The 'done' window is much narrower when fat levels are relatively low--think of the difference between the wide window one has with a fatty butt vs the narrower window one has with brisket.
Overcooked ribs are dry and tough; undercooked are usually tough but not really dry--they're more like chewy and not moist enough--there's a sense that you're trying to chew through connective tissue that has not broken down; with overcooked the sense is more like you're chewing dry meat fibers. (Does this make sense?)
"Hammy", to me, suggests meat that was rubbed too far in advance of cooktime, meat that was 'enhanced', or both. It also suggests the possibility of a high salt ratio in general.
"Burnt" suggests too high a cooking temp for the meat and/or the rub, or too long at whatever temp (though that's less common at lower temps).
What temp was your cooker at grate or lid? Do you happen to have a pic of the ribs before cooking? If not, how did the fat look--do you recall?