I'm wondering about the security against attacks - as far as I've understod, there is some PoW going on, every node in the network does efficiently mine their own transactions and also has to verify (at least) the same amount of transactions made by others. And the difficulty rating is constant.

To me, it seems pretty obvious that a future super computer (doesn't need to be a quantum computer) should be able to do some "51%-attack" and rewrite the history.

Also, the though that embedded devices with limited CPU resources and limited batteries should be doing PoW ... that sounds rather inelegant to me.