If you buy Ledger from an unknown seller or if your your seed can be accessed by someone else besides you, you can instantly lose your funds. On Reddit, we can keep seeing users who either leave their seed at home or save it in their email and then some users who have bought it from a friend/seller online and they get their wallets hacked. Trezor is the same in this case and these wallets are not safe as you have the seed which can be lost as well. They should have some more security features so that even if someone gets access to the seed, funds can still be not moved.