I agree, Niche's are great and have fun with them. I did 2, one is very large and holds lots of stuff but I keep it to a minimum it also doubles as a foot rest when shaving and the other is long and skinny with 4 shelves that are pretty much out of water splash zone. The top shelf is for a stack of wash cloths that are stocked at all times, never will I have to hear the pounding on the walls followed by "Can you bring me a wash cloth?" Next shelf for Face wash like Noxema and 3rd shelf is for his and hers razors the bottom shelf is a bar of soap that sits on a folded clean dry washcloth so that the soap dries nice and no soapy mess to ever clean up. Have fun with it! And build in the organization.

DH here. Conventional wood framing, then hardbacker, which adjoined to the rest of the hardbacker that formed the shower walls. In other words, the niche is simply part of the wall. Warning -- if possible, frame the niche at least 2 inches larger than your desired final niche opening dimensions, and don't seal with Hardibacker, nor sparkle with thin-set, until your tiler does the actual tile layout measurements. Depending upon where the tile lines lay, you may want to add layers of Hardibacker to fill so that you don't have to notch tiles at the corners. Then, we waterproofed the entire hardbacker surface (walls and nook) using three coats of Laticrete's Hydroban product (rolls on like thick goopy paint, dries to form a rubber-like seal). According to our tiler, if/when water gets behind the tile (wall or nook), it slides down the Hydroban and exits into the tub. Had to shim walls to make sure the Inside edge of the Hardibacker overhangs the raised lip of the tub, so that water sliding down behind the tile exits inside the lip of the tub. Apparently, an alternative to shimming the studs is to notch the framing so the tub lip sinks into the framing by 1/4", which lets the Hardibacker's inside edge sit inside the lip of the tub. That also helps the tiler bring the tile down inside the tub lip without angling the last row of tile in to clear the lip.

I'd tend to say that if the GC is admitting they're upside down and offering a half-assed fix, they're upside down. Nailers, these days, are on both the top and bottom, usually. They also provide stability, even if the cabinet is directional. Now. If I, a rank amateur, can put up my cabinets and take a single one down in a 16' run (when I find or make a better one) and replace it with another, these [ahem] professionals can, too. I'm certain he sees his "fix" cheaper than fighting with people he's probably already paid to come back and fix their stupid work. I cannot believe they didn't notice a finished top IF they were installing crown molding or light rail. Did they think no one would notice? This is horse crap and do NOT accept this. bmorpanic, the OP already said that her cabinets' undersides were finished. So probably this "fix" isn't going to be the factory finish she bought and paid for. And I think I keep getting fed lines of bull. Please don't say OK to this. I have spent 6 years fixing and rebuilding the House of Good Enough For Now. Nothing is good enough that RIGHT. You'll end up with something falling apart and your GC will be long gone.

Why not both? If you only do the corner shelf, you can't sit on it because it will have shampoo and soap on it. Use the handmade tile for an accent that includes a niche, and add a small white corner shelf for use with leg shaving or to sit. For something as small as the photo above, I would use a solid surface rather than tile so you don't need to be concerned about the edges. I would not make it a big feature. You do need to plan ahead so the shelf has support in the wall.