There are currently some right-click actions with shears on mobs that dispensers containing them cannot do, namely the following:

• Removing pumpkins from snow golems
• Shearing red and brown mooshrooms

Dispensers also cannot shear placed pumpkins, however this is covered in a separate issue which has been marked as WAI (although not by Mojang, so it may be worth looking over); see MC-148448.

Relates to MC-39821, MC-165196, MC-164345, MC-131061