My 2 cents-
Well from my slight bias as a beguiler, I know that it seems a little op if it also silences users but from what I seen, the target has always been able to use spells when mounted on the pig (target has always been able to use fireball, ect from when I used it on them), they are also able to use melee against us too. While I dislike the vanish after a hit since it becomes like root except the target is still mobile (pig moves unlike root and other similar skills)
From my side the target is free to moves after the 6 seconds, there has no indication that they suffer what megan had explained. The exception is the collusion bug where the pig send the target to the void.
___
Suggestive comments-
It is OP in the sense that they target is subjected to movements they they do not control, which would usually give time for the caster to catch up but the same is said for root and similar skills. These skills that are associated with holding a target in place are constantly in debates. The usage of these OP have been brought up numbers of times. I would compare piggify more closely too "Confuse" from Theif since the target is still mobile on the pig only that they are subject to the pig's random movement much like how confuse work.
I agree that it's OP in the sense that it lowers the chance for a target to get away but this kind of skill is usually to give an advantage in a short time period so that the caster can use their skills while they have the upper hand just like disarm for warriors and confuse for theifs or root for other classes. To counter act the spam of the usage they have longer cooldowns than attack skills and Piggify is 25 secs. CD while the skill last 6 second. Which is why I believe it's more balanced than root's 15 sec CD which last 4 secs.
Instead of moving this skill towards the same category as root, it would be better to associated it to the skill "Confuse" used by theifs.
Confuse-
"For 10 seconds, target moves 2.2 blocks in a random direction every 0.75 seconds."
the pig from piggify causes the user to move in a similar fashion only "weaker" as the movement is governed by how pigs act. It should be more similar to that instead of similar vanishing after an attack.