If their servers go down, you won't be able to view your balance and you won't be able to create outgoing transactions.
You can. Trezor is BIP39 compliant which means you can easily import your seeds to another wallet which would allow you to spend the funds with the same addresses that was in your Trezor HD wallet.
They also know every UTXO you've ever sent or received. You couldn't develop a better way to spy on Bitcoin users if you tried.
Can't dispute this. I would mitigate it by using Electrum instead of their servers though.