Mass Exit Attacks are possible on the Lightning Network, according to this study.

Cosimo Sguanci and Anastasios Sidiropoulos of the University of Illinois conducted this study indicating that it's possible to attack the Lightning Network using what they call a "mass exit attack". They say the attack would exploit timing limitations on the Lightning Network to force a mass-exit event. It could take the form of a zombie attack or a mass double-spend attack.

Now, I'm not saying that the Lightning Network is bad. Just that these researchers found a possible flaw that could be used to do damage.