The animation aborts because druid of the claw and the druid of the claw (bear form) use the exact same model. If you look at the different animations you can see that the "alternate" animations are the druid in bear form.
When it changes to a dryad, the model changes instantly, but the dryad doesn't have a morph animation, so you don't see anything.
Also, if you want to retain abilities, there is a line of JASS code that will make a spell permanent so that the druid can retain its abilities after a morph.
If you don't want the dryad abilities, make a custom unit based on the dryad and get rid of its abilities and then have the druid morph into that instead.
Hope that was helpful!