Any advanced effects are prone to stuttering in the animation loop. Javascript is bad with this, but OS level stuff can stutter too.
Stuttering can be destructive to User confidence as well.
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Also with morphing there is a problem of Mystery Meat[1] navigation. In this depicted example the Morph is an alternative to another mystery meat, yet there are many problems that could have both cause and effect permanently visible.
I never saw OS X animations stutter. Sure, Expose drops frames now and then (dropping frames is the sensible approach, not displaying every frame and introducing stutter), but that's ok. That makes the animation jerky but just as quick and reduces the impression of stutter.
I think it is very plausible that seamless and low key animations can increase usability. Sure, user testing would be needed to verify that, but it seems intuitive enough to me. There are technical problems, sure, and stuttering animations should be avoided, but that's no reason to reject morphing out of hand.
Stuttering can be destructive to User confidence as well.
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Also with morphing there is a problem of Mystery Meat[1] navigation. In this depicted example the Morph is an alternative to another mystery meat, yet there are many problems that could have both cause and effect permanently visible.
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[1] http://www.webpagesthatsuck.com/mysterymeatnavigation.html