That's an excellent question. So far, there hasn't been any research conducted that would verify the level of NANO's security against external attacks. My guess is that NANO is much more vulnerable against spam attacks than other cryptocurrencies on the market because the network has a fee-less structure. The attacker could just flood the network with lots of transactions in order to diminish its performance. This hasn't happened yet, probably because almost everybody is focused on the big players on the market.

One thing for sure is that NANO is cheaper and faster to use than traditional cryptocurrencies such as Bitcoin and Ethereum. But it hasn't gained traction probably because of the lack of marketing/promotion. With low mainstream adoption, the coin will remain under the scope of many serious investors and traders alike. At least, the NANO project is being actively developed and maintained by the community. Its open source design will allow anyone to improve the project or build one from scratch. Remember, it's not about crypto but rather the technology underpinning it. Blockchain is what has made all of this possible. No matter if it's NANO, IOTA, XLM, Ethereum, or Bitcoin, all of these cryptocurrencies serve one common goal. And that is the decentralization of money for everyone. Whenever NANO has a bright future or not, it's yet to be seen. Just my thoughts