The problem arises during times of high volatility in that you might buy BTC worth $300 at 1:00pm, initiate a transaction to a vendor at 1:05pm and yet even in that short of a period of time the price of BTC could have dropped by 5% and the BTC price recorded on the transaction would show $285 and not $300. Potentially, this could be enough for the vendor to tell the customer they need to send additional funds to cover the missing $15.
Ideally, the vendor would specify an exact BTC price like 0.003099 BTC for customers wishing to pay with BTC. That would eliminate any questions, but then you'll run into customers who may delay sending the BTC in the hopes the price will drop a bit and they can buy the BTC at a discount. Of course, if the price goes up while they wait, they'll be bitching and moaning about that, too.
I'm sure I'm not telling you anything you don't already know, but I hope some of the others who might read this take it as a caution to transact in stablecoins such as USDT or USDC over Ethereum.