In my experience, it would make more sense that unburrow would be delayed until the turn after it burrowed. I cite Phoenix as the reason why. Mutated creatures can use abilities right away because the creature was already active on the board, albeit in a different form; the same deal goes for Mitosis. Phoenix is a special case, and I think that it is more similar to Skittish than Mutation. If you kill Phoenix, you can use Rebirth the next turn. If the enemy kills Phoenix, you have to wait until your next turn. In addition, if unburrow was present at the start of the turn, you would lose no attack power each turn provided you unburrow promptly, and it would be functionally identical to Immortality with the caveat that it is vulnerable to mass CC. I think that Skittish should have a drawback, in the same way that the Phoenix skill has a drawback, both by making you wait a turn to regain lost attack.