Yes that's a poor job. What did they do for waterproofing? If there is no waterproofing behind the tile, then it's a complete do over.

It even looks like pieces of tile are missing.

fyi: GC defended tile setter by saying:

but ending on larger full tiles are part of the design also we were able to avoid slivers in and around the niche without a cut in the corner. In some corners it’s impossible to avoid small cuts and not pass that around to another corner to make a smaller cut. It’s never preferred but is sometimes a reality of the math of the tile. Our regular process is the designer will layout the tile after they’ve met with the homeowner to go over smaller details like what corner the smaller cut goes into. The real question is what is the remedy. Other than caulk (which is necessary at all change of plains) there isn’t much of a physical remedy. What did you have in mind?

He's not wrong that there are sometimes tradeoffs to be made when laying out longer tile in a bond pattern. But the argument doesn't make much sense here--looking at your photos, if they had shifted the pattern over slightly to eliminate the slivers on the right side of the niche wall, it would have benefited the smallest pieces at the edge of the niche. And on the wall with the plumbing, they should have started with whole tiles at the corner. I can't see the other end of that wall in your photos so not sure if that would have left you with slivers on the other side. But based on the layout visible here, it looks like that wall turns a corner so a sliver on the other end would have been preferable visually since that wall seems to be more visible than the smaller corner on the left side of the plumbing, which you'll only see from inside the shower if you happen to be looking out. The wall to the right of the plumbing will be visible from the main bathroom, it looks like. So again, tradeoffs might have been necessary, but it doesn't sound like they consulted you on it. And honestly, with tile this long you can also fudge it a bit and not have the pieces wrap the corners perfectly proportionally--e.g., you don't have to go from a 6" piece on one wall to a 2" on the next simply because it's 8" tile; you can wrap to a 2.5" or a 3" piece if it eliminates slivers and your eye will think it's right because it's close enough.

Not sure there are many remedies at this point, unfortunately. Caulk will probably hide the very skinniest slivers, but not the bigger ones. You could potentially put a corner shelf in to cover some of it, but it doesn't look like you need one with the niche there. Fixing it in any more substantial way would mean ripping it all out (and probably starting from scratch since I doubt you could do that and keep the waterproofing intact).

I would use caulk that matches the tile instead of the standard matching it to the grout--unless of course your grout also matches the tile.

I'd definitely want the missing tile put in.

They are willing to give us a credit, we're not sure what % of the total labor is worth asking for.

I think it depends on what you plan to do with the credit--if you're going to have it fixed, that would be a lot more than if it's just to make you feel better about having to live with it. You could get a quote from an area tiler to see how much they'd charge to retile and ask for that. Depending on what the waterproofing situation underneath is, you might also be able to remove and replace some but not all tiles to address it for some of those rows (e.g., to the right of the niche, you could potentially remove the slivers and use a slightly longer tile at the edge of the niche without your eye picking up the difference if this won't compromise the waterproofing membrane--but it may). Similarly, for the wall where you only have the very narrow slivers, if you can remove tiles, you might be able to space the tiles slightly more for the length of the wall to pick up that space and eliminate the need for a sliver. Would still not be a perfect solution, but to me that would look better. All moot if you can't remove the tiles without starting over, though.

On a separate note, also be careful with the grout choice on your hex tile. The way it's installed, you can see the edges of the sheets--a darker grout will accentuate that, but a lighter grout will help mask it. (It's a common problem with hex, but the wrong grout color can make it worse.)

He's right that much of the work in the corner will be hidden by a well done caulk bead. The key here being a "well done" bead.

My bigger gripe would be the horizontal lines not lining up through the corner.

They aren't off too much but there is no reason for that. Experienced tile setters understand how to lay-out tile and set it so your horizontal lines are all perfect around inside and outside corners. That's a very basic part of the starting process.

As other commenters have noted, when I see basic cosmetic issues like this it sets off the alarm bells that you have an inexperienced or unskilled tile setter doing your job, so then the next question becomes: if he can't lay out tile properly what makes you think he can waterproof it properly?

With caulk, some of the imperfections are hidden for sure but this is just the wall with the least egregious issues

Too fat for a caulk bead. White gloss solid surface cove molding please.