Also, there is a certain unavoidable irony in suggesting the way to scale bitcoin is to avoid using it.
Another disadvantage is that if you control your key, you must be online. Like, if you run an HTTP server, you must have a machine. With plain Bitcoin you can spend to a pubkey and it doesn't have to be online
To summarize.
1. Attacks are extremely unlikely in the first place.
2. Watchtowers can prevent them if they do happen.
3. Watchtowers are trustless. They in no way resemble banks.
4. Without a watchtower you are not required to be always online to be secure.
The channel state appears on the blockchain once either party closes the channel.
Think of it like opening a secured credit card with cash. Faster and more convenient to use, since you can now use the credit card networks instead of the cash, but requires tying up the funds you want to use to pay with.
Also, the merchant/recipient has to be able to accept the payments, and the software is new / in beta still.