tl;dr sometimes there’s a problem, sometimes not. Depends on who you ask and what the product is.

Depends on the shroom and intended preparation. For example, if you’re going to be searing buttons or porcinis, you shouldn’t wash them with water. Mushrooms, to a varying extent, are usually highly absorbent. The extra water in the shroom will make it harder to get a nice sear, and in my opinion, will mess with the texture and flavor of the fruit. Just like with cooking meat, a lower water content results in concentration of flavor and easier, nicer sears.

In contrast, if you’re going to be cooking mushrooms down for a long time, and don’t need to have an intact sear with crispy bits, you can wash with water. For example, if you were going to be making duxelle, cooking then down to make jam, grinding them for aminos, or throwing them in a stew, water is fine. I’ve had chefs who say otherwise, but it’s really quite hard to tell the difference in the final product sometimes. If you need to process a lot of mushrooms that don’t need searing, it’s fine to wash them with water.

There are also certain shrooms that absolutely require water washing. The first one that comes to mind for me is black trumpets, though there are others. They grow near pine trees, and usually form around fallen needles and other detritus. There will be dirt, needles, moss, and twigs INSIDE the trumpet. They’re delicious as they are expensive, but a pain in the ass to clean. You generally need to tear them in half down the middle, then spray/rinse them with cold water, drain, and repeat several times until they’re clean. Then pat dry. Luckily, as mushrooms go, they aren’t that absorbent. This makes them great for applications like mushroom jams. They also work very nicely simply cooked down with some herbs and dry white wine, then folded in with pasta. No sear necessary. You can’t even really see maillard if it forms anyways since the fruits are so dark to begin with.

That was probably too much info but hey, sue me I like mushrooms.