Apart from price fluctuations, the process itself has some additional costs.
One cost is the network fees you have to pay to create transactions in bitcoin and monero networks that can add up.
The other cost is the exchange fee you have to pay the platform like Bisq that you are using.
The last cost I can think of is the spread. Sometimes if the trading volume is low, there is a spread in the orderbooks between the top sell order and the top buy order. So even if price remains the same and you buy that altcoin and sell it immediately you still lose some money because of the spread.

The other disadvantage I can think of is the complication of this method. Someone who has never used Bisq before or have monero wallet/used monero will have to learn how to do these things first which is time consuming and could be difficult for some people.