Since Ledger/Trezor is not available in certain countries or there could be trust issues there, with this kind of setup you should be basically risk free.
If that would be the case, I'd rather use an old netbook, they're cheap and much more versatile. Take out the Wifi module if needed, and install anything you want offline.
On the other hand, this hind of hardware may be safe since you flash it yourself with the software you probably check first.
Again: I'd trust a standard netbook with standard Linux distribution more,
especially when kept offline. If
USB cables can be used to
access your computer, how long will it takes before "offline" devices come with their own hidden internet connection?