If the USB drive is a live OS and the private keys or seed are not saved in persistent storage, the solution can work.
I wouldn't really call that a wallet though, that's just a client (such as Electrum). You would need some other method of storing your seed and transporting it around to have whenever you want to transfer some coins.
You are right, the ledger nano x is portable and can can be use with a mobile phone through bluetooth. Although, if you use the ledger live app, you are limited to 23 cryptocurrencies.
The Nano S can also be used with Android phones (but not Apple phones) via a USB cable. Further, you are limited only to 23 apps being installed at the same time. You can freely uninstall and reinstall apps without losing your coins, so you in reality you can store as many coins as the Ledger supports.
From what I know (please let me know if I'm wrong) there were movies showing that it's not that difficult to "hack into" a hardware wallet if one has it physically.
There have certainly been attacks proven to be possible against Trezor wallets. Even wallets which have no known attacks against them shouldn't be assumed to be 100% safe for ever more. As you say, if one of my hardware wallets was stolen, I'd be recovering from back ups and transferring to a new wallet within a few hours.