Another refreshing sign of competence from the new administration. Yellen would be well aware of how destabilizing deflationary cryptocurrencies could become to the macro economy.

The more a cryptocoin is used as a savings asset, the more it gets tied to the real economy, the more it can become destabilizing.

When gold had too much ties to the real economy, it caused enough instability to cause the great depression.

This happens because people, businesses and banks start hording tokens instead of investing in building and maintaining the production capacity of consumables so the ratio of value of tokens meant to buy consumables vs the actual production of consumables gets out of wack.

When the music stops, people realize that other people are sitting on tons of seemingly valuable tokens but stuff to buy with them is getting scarce because capacity is not being maintained, they reverse their trade, prices of stuff go up, volatility ensues, and the destructive cycle resets.

Too much savings being tied to tokens instead of assets that create real value is an economy wide coordination failure, a terrible Nash equilibrium. It can be hard to solve because it's a prisoner's dilemma where the first people to invest in real production are disadvantaged so everyone hoards tokens instead (the gridlock in the production market this causes ends up hurting a lot more overall).

This is what central banks and sound currencies are designed to prevent (as Yellen would know). We don't want cryptocoins to cause another great depression like gold tied currencies did almost a hundred years ago.