I think that LN will eventually be an important part of the overall ecosystem, but there are some downsides:

First, the biggest downside of LN is that the recipient has to have their LN node online and listening at the time of the transaction in order for the transaction to occur. This is fine for payments to web stores etc., but I think that it will largely preclude usage of LN for more peer-to-peer transactions (eg. forum trades). It would not be good if people were expected to use only LN, since that would result in almost all individuals using trusted-third-party wallets in order to accept payments.

Second, LN often does not integrate easily with existing BTC payment systems. For example, I would have to almost completely rewrite the bitcointalk.org payments system in order to make it work with LN. (Someday I'll do it, but not soon.)

Third, it's possible that the network will end up excessively centralized. If 90% of LN channel-value goes through a small handful of nodes, then that would be a real problem. That sort of centralization could lead to: 1) a lot of people getting their funds locked up for a long time; 2) possibly a slippery-slope to further centralization, eg. "gatekeepers"; and 3) possibly even losses if too many unilateral channel-closes are necessary at one time network-wide. However, the degree of centralization in your channels is completely controllable at your end. If you want to avoid channels which go through a certain highly-popular node, you can do so (possibly at higher cost), and then you will be immune to problems related to that node. You can never be forced to accept centralization with LN. So ultimately this ends up being a software problem of properly informing end-users of centralization and risk. Depending on how the network ends up looking, this might or might not be a difficult problem to solve.

In a few years, the majority of transactions will probably go through LN, but I think that it's currently a bit over-hyped. LN can only be part of the overall picture, and it'll take quite some time to figure out how to get it to fit smoothly and safely into everyone's lives.