Scaling is a big problem, but can probably be solved in the (far) future. The last spam attack showed that at 30 tps a lot of weak nodes had problems and the network became near unusable for many. Even strong nodes will have issues past 100 tps. Afaik there hasn't been any meaningful research into things like sharding and significant scaling beyond just better hardware.

Another downside of nano is that it requires 2 blocks per transaction: send + receive. This can cause issues in times of congestion, like tokens being "stuck" between sender and recipient. Also some slightly misleading tps stats.

I'm not 100% sure on my next points, but I'd love to be corrected. The nano foundation has very little funding left compared to many other projects. Distribution was somewhat fair, but what happends if their funds run dry before there is enough regulation around crypto payments? Payments are an obvious use case for DLTs, but they also take a very long time to get regulatory clarity.

The fact that they can have 300ms confirmations, leads me to believe that nodes have to message each other node directly instead of using some form of gossip. This can cause a lot of problems with increasing amounts of voting nodes (~60 atm). Not sure of every spectating node gets the same messages.