Definitely one issue with crypto-currencies today is that they have volatile pricing which makes them difficult to use in commerce. There are several projects working to use smart contracts in order to peg the value of the coin to currencies like USD, EUR, etc. As liquidity increases in decentralized exchange formats, such as atomic swaps and DEX applications, there will be enough volume to properly manage these smart contracts to peg coins to fiat currencies.

A unit of account from my understanding simply means that other people are willing to price their goods/services in your currency. This is a by-product of people participating and using the token, and it being stable enough. So once there is a "stablecoin" that gains traction, this will surely follow.