If the niche is in the spray pattern of the shower head or within the rebound area of water bouncing of of the bather's body, yes, the niche will see water. And the grout will get wet. Just like the grout on your shower floor.
I build niches with a topical waterproofing, so the only things that can get "wet" are the tile, the grout, and the thinset that bonds the tile to the membrane. They'll dry out. And even though they can get wet, water can't penetrate through to get to the shower's structure or substrate due to the waterproof membrane right behind the tile.
With a properly detailed niche, you shouldn't get any water damage.
Goop? As cindywhitall wrote, if your niche grout is wet, and you use lotions and potions that tend to gunk things up? Then your grout can get a bit nasty. But it's nothing a once-in-a-while scrubbing won't cure.
Do make sure the shelving in the niche is sloped to drain into the shower.
Do make sure that your tiled curb is sloped to drain into the shower. With proper detailing, no water will get through to the curb structure below the membrane. I build my curbs from mud, so even if water got through to the mud it won't cause any swelling. A curb with a wood core? If wet, the wood may swell and blow the curb apart.
Be very careful with penetrations through the curb tile or grout. Any penetration must be sealed/waterproofed.
It's not uncommon for a beautifully built and properly detailed shower to be undone by a glass installer driving screws for glass clips or hinges through the curb membrane, not properly sealing it, and allowing a path for water into the shower structure.
Good luck with your project!
Q
Shower Photo
Q