I agree with the general gist but 2 years ago I made a strawberry bed where the potatoes had been a few months earlier. I dug in plenty of compost and leafmould and the plants have done really well. So it can be OK. Although the potatoes had only been there that once. I know that for certain because it was a field before. Just depends on your attitude to risk I suppose, and whether there's a viable alternative. Don't know about mustard, but why not try it. It'll do good anyway.