Dress it up or call it whatever you want, but there have been horrible problems with it in the past, and nothing has been done to prevent them happening in the future. A problem is a problem regardless of what type of problem you want to categorize it is.

You can Google this, but the specifics might be hard to get at without specific terminology. Apologies if this is long.

Salt water is not just salt and water. It's a lot of dissolved things at a low concentration, so when you purify it, you can remove the water, but are left with everything else to deal with.

The byproduct is called "brine", and it's basically a highly saline slush with all the heavy metals and now microplastics that humans have fucked the oceans with.

You can't even really turn it into anything else in most cases (locations vary on purity) because after you remove all the water you have things like mercury, lithium, chlorine...whatever other minerals the locale has leeched into the water by natural process or dumping. Due to the processing it's all HOT and volatile, so you cant just let it sit there, you have to put it some place.

You can't just dump it back into the ocean because then you're making the ocean TOO salty and killing everything. You can't put it into a landfill because then you're just building a toxic waste dump. You have to essentially do what we do with nuclear waste and bury it in a vault because there is no good use for it (maybe this can change in the future), and the chances it will just leech out into the world and create an ecological disaster is insanely high.

For every gallon of ocean water you desalinate, you have something like an ounce of this leftover brine. Do that a few million gallons a day, and you have quite a lot to deal with.

This isn't even taking into account all the methane producing living things you're killing by filtering out before you even get to the actual desalination steps, which is a whole other thing.

Tldr: it's not a good way to get water.

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.