I think that it’s more that the viewer normally gets to see things from the view of the good guys, and suddenly revealing that someone is actually someone else shape-shifted — which doesn’t work if the viewer has been following along with the good guys — doesn’t work as well.
Morph, from X-Men.
https://en.wikipedia.org/wiki/Morph_(X-Men:_The_Animated_Series)
I think that it’s more that the viewer normally gets to see things from the view of the good guys, and suddenly revealing that someone is actually someone else shape-shifted — which doesn’t work if the viewer has been following along with the good guys — doesn’t work as well.