Cedar ribs would be fragile. Birch bark canoes are traditionally ribbed with cedar, so who am I to say no? You'd have to size up their thickness, which might nullify the weight advantage of using cedar in the first place. They'd probably bend easier than re-soaked oak, too.
If you're going to build one like Brian Schulz's (which is basically what I do) be aware that artificial sinew will dig into softwood like a knife if it's wrapped tight enough.
I, Brian, Anders Thygesen in Norway, and probably Robert Morris use white oak for ribs and I haven't learned anything that makes me want to do different. There are some terrific youtube resources out there. Of those mentioned, Anders knows an almost unbelievable amount about how SOFs behave in the water.