I don't know myself if it was stated that the Echo would grant us protection only against primal tempering. If that wasn't stated, then the same Echo is what also prevents the Ascians from using our bodies as a flesh tuxedo. the Scions don't have the Echo, except our dear waifu Minfilia, so there you got another reason of why Thancred could get posessed.

We, the main adventurer of the story, have a strong will to not succumb to evil so easily.