I don't understand how extra skills = extra liability. There are always cases where a little knowledge, but not enough, is dangerous. However, there are also many cases where mistakes are made for many other reasons. The nurse needs to stay within the scope of practice of the licensed role they are fulfilling - if at the bedside, then it's the RN scope.
Certainly, the immediate transition - from NP back to bedside - would be more risky, but a transition period is always more risky - such as new grads to practicing RNs.