I've been washing nappies in soap nuts for 14 months!
You never have to worry about detergent residues, allergies, etc.
I would agree they do not get out stains. If you use liners, line dry in the sun, or just don't care about stains, this is not a problem.
I pre-soak in vinegar if there are nasty poo nappies, and add water softener to the wash since I have hard water. I do have a front loader machine (High Efficiency). It's possible a traditional machine might need more than the recommended number of soap nuts?
I do run the occasional wash with detergent and the occasional 90 degree wash, which I believe is recommended anyway. (you should also do the occasional 90 to keep your machine from getting smelly, this may be why pp had problems with a smelly machine)
I recently noticed my nappies are getting a soap nut smell so am questioning whether soap nut residue is possible after all. With that in mind I've started doing more detergent washes, and reduced the number of soap nuts I've been using to even less than it says on the package. If you have been using detergents it could be a good idea to use soap nuts for a while to get all the detergent residue out.
Give the nappies a test smell when you take them out. You should figure out whether they are clean or not!