Bitcoin price goes up and down all the time. If someone buys something costing 10$, the next day it may cost less or more. So it's a risk for the seller.
It is a risk only if seller not selling his Bitcoin instantly to local currency and decide to hodl.
In past i was thinking all sellers should adopt Bitcoin. I don't think that now, Bitcoin is like digital gold/store of value, it doesn't suit purpose of fast and huge volume transactions, lighting network build on top of Bitcoin can do that, not Bitcoin directly. Higher layer is needed for shops to use Bitcoin.