If you and I have a lightning node and a channel between us, and I send 1 BTC to you, this transaction is NOT secured by the bitcoin network, nor any similar technology. It's basically just a regular ledger that my node and yours keep track of (there's a little more cryptography in place, but not much more).

Even worse, if you saw my transaction, then sent me 1BTC worth of goods, then your node becomes offline and I forcefully close the channel with the original balance, I still have my 1BTC and there is no proof anywhere except your node that the transaction ever happened. So no security, unless both nodes are online.

Finally, unless you open direct channels with everyone you want to transact with, you will rely on a 3rd party node that does have such a channel. In practice, this likely means you will use one of a few major nodes that you trust to stay online and not refuse transactions, so the payments are in effect highly centralized.