Trezor is also
open source. All Trezor does is keep your private keys offline and sign transactions. Both Electrum and Multibit HD are lite wallets. Electrum has the option of cold storage using two computers like Armory, or you can use Trezor or Ledger with Electrum to sign.
Indeed. But how do I know for sure that nobody modified the firmware on the trezor. For example somthing like this:
privkey = SHA256(privkey & 0x000FFFFFFFFF); // Leave only 36 bits of entropy
that will be exceedingly difficult to detect, but the culprit could harvest all Trezor wallets a year later.
Yes, I know, the same could be done in the precompiled binary of a wallet. At least in Armory, I could generate the keys by shuffling a deck of cards.
But I am probably being unduly paranoid.