Despite significant growth in recent years, the Lightning Network still faces challenges to overcome if it wants to solve bitcoin’s scalability issues. The most demanding issue is security. Because nodes on the Lightning Network are required to always be online, they become more vulnerable to attacks. And while the network aims to reduce fees incurred from processing transactions on bitcoin’s main network, it includes its own set of additional costs for opening and closing channels, along with routing fees. These are issues that will likely be solved with time, as its technology develops and becomes fully optimized.
Source: https://www.coindesk.com/learn/what-is-bitcoins-lightning-network/