Yes, there's a possibility that the transaction gets reversed when it doesn't get confirmed for a long time. I don't know how long but when I experienced it for the first time it lasted for a couple of days before it disappeared on the explorer.
This happened to me too. One of my transactions got reversed thankfully since I thought that this was going to be a major problem of some sort. Default fee is the optimal solution to these problems which is why it is not recommended to lower the fees.
Instead, increasing the fees is encouraged if you wish to get your TX confirmed quickly.