The biggest problem of block-chain and cryptocurrency, in general, is scalability, transaction cost not much, to be honest. and those IOTA or other “fast block-chain” like ripple fans, we aren’t just talking about sending coins because most of the coins aren’t meant just for that, they are also meant for micro-transactions between machines or smart contract that do thousands of tx per second or maybe even much more. and we haven’t tested those numbers yet. and when we have we sort of lost. so yea, Scalability is a big problem here.