So, it'd lose the buffs when it unburrows and therefore is not doubled when it does.
Exactly. This will be most beneficial to antlion and devourers since they will get the largest percentage buff. It will not help unburrowed creatures though.
E.g. a burrowed shrieker does 5 or 6 damage, unburrowed it will deal 8 as normal. A burrowed antlion will do 2 or 3, unburrowed it will only do 2. As you can see, the smaller creatures get the most love
so hopefully we will start seeing some antlion usage. (antlion fractal anyone?)
Starting the creatures out burrowed will also provide an immense tactical advantage since CC will be relegated to shields.
Starting off burrowed will also allow the creatures to be employed while flooding is in force. This was previously only possible via quintescence (or annubis / aether nymph).
The relatively low shield bypass chance should hopefully keep from overshadowing gravity's momentum card.