@Roberto Julio Azar It is 100% secure because trezor generates and holds the keys at all times, they never leave the device. The online interface is the beta part and Trezor is a VERY professional company. If the interface goes down however, you can just run electrum-dash and get your coins out anyway, its built to a standard. You can also use the same Trezor for multiple coins at the same time.
In all fairness I guess nothing is really 100% secure, but if Trezor is ever hacked, they will be going after the exchanges's coins before ours.
I hope that answers your questions
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Pablo.