I've never tried it for cold smoking, but I did throw a chunk on the UDS once when I was doing a couple of briskets. It was good, but is definitely strong. I wouldn't recommend it for anything mild.
Walnut has an intense flavor, and yes it can be bitter if too much is used, but can still be used for smoking meats. You should mix a small amount with other wood(s). When we have used it for brisket our mix ratio is around 10% walnut and 90% other wood(s)