Salt brine is highly corrosive to anything in contact with it so it is difficult to move, and in high concentrations wreaks absolute havoc in surrounding ecosystems. There’s a reason the phrase ‘salting the earth’ doesn’t have a good connotation. Desalination on a small scale might not seem like an issue but when dealing with the waste at scale becomes a bigger issue than even the energy cost to perform.
One part oxygen, two parts hydrogen and squeeze.