Personally I've had better luck with thin CA and coffee grounds than epoxy and coffee grounds. In my experience the CA not only soaks into the grounds, but also the wood around it, creating a stronger bond. The same solution using epoxy doesn't bond as well with the surrounding wood and I've had more inlaces turn loose when hit with a gouge than I've been able to smooth and finish. Plus, it is much easier to fill tiny cracks with CA than with epoxy - just rub some near-powder coffee grounds (or powdered metal/stone) into the crack and put just enough CA on it to soak in, wait 2 minutes, sand a touch and you're back in business.
It will soak in and stain wood where you don't want it to so be careful how much you use or seal the piece with shellac/wax/DO/lacquer/etc so it doesn't soak in. Thin CA will cure almost instantly when it meets coffee grounds, so on larger voids I will make a dam around the void with coffee grounds and if the CA runs it will stop and harden as soon as it touches the dam. And if you have some uncured CA sitting on top of your fill you can sprinkle some coffee on it and it will cure instantly.
I also prefer the more consistent density and appearance of coffee grounds with CA over (untinted) epoxy. It looks more natural to me, but appearance is subjective. In the end you should use what you're familiar with and what works best for you. In all fairness I've only used QuickCure 5-minute epoxy from Lee Valley. The slow-cure stuff may work better, but this is what I have and my experience with it.
If I think about it tonight I will take some close-ups of some fill jobs I've done and post them up.