Each slab would have it's own shortcomings, so all will not be the same. So the stone yard saying that they had not had any other problems may be correct, however yours may be the only problem.

I think the seaming should look better. Also where is this section located? If it is in a high wear position I would want it corrected.

Your installer having to buy all new tools could be correct due to brittleness of the product. I do not think there is a clear answer either way. Buying a piece sourced from Taiwan may be the culprit, so all involved are at fault.

It's located on the peninsula where bar stools will be.

I have wondered about where the slab is sourced from! I assumed it was sourced from Italy being name Arabescato Carrara but after our fabricator complained I found out it was sourced from Taiwan.

Hi. We have had the same issue with the arabascato slab we originally chose for our kitchen island.

Our fabricator also mentioned to us that the stone was like "sugar" and that it crumbled every time he tried cutting it. He went ahead and installed it for us anyway, but after living with it for only a few weeks the edges kept chipping and crumbling, and there was always white dust on our floor in that area of my kitchen. Luckily for us our fabricator was amazing - after we complained to him about it he admitted that he shouldn't have let us use this type of granite since it's so brittle and so he let us choose a new slab to replace it.

We ended up with a beautiful slab of super white which is really strong and is treating us quite well so far!

This is not typically acceptable. I work at a slab fabrication and tile showroom, and we would never install a job this way. Marble comes from all over the world, some being better quality than others (which plays a part in the price- cheap marble is not usually good marble). However, since these materials are natural stone out of our earth, there has been history of having a "bad slab" in the bunch every so often. We've had issues before where we purchased certain slabs all from the same lot, but only had brittle/breaking issues with one or two out of the lot. Typically, the vendor has been very accommodating in replacing the "bad" slab, because unfortunately things do happen occasionally when working with natural stone. HOWEVER, those seams are NOT from a bad slab. They just look like they were poorly executed/installed.

Alexis, how can you tell that it isn't a bad slab by the picture? What would the seam look like if it were a bad slab?

I cannot tell from the photo of it is a bad slab or not. That usually unfolds when we start cutting and the materials starts to crumble like sand when it hits the saw blade. The seams just seem awkward being placed like that, and the center strip of marble doesn't look like it matches the rest of the countertops. Maybe that's just the photo angle though? Anyways, seams should be straight and should be the same thickness all the way throughout. The photo looks like two seams are different thicknesses, and the left one gets larger as you get towards the post.
Is this the underside of the countertops? If so, the seam getting bigger towards the post isn't a big deal as long as it's not visible like that on the top side

The whole poor installation started with a design with a 2x4 in the middle of a counter. It was compounded by a poor choice in stone and a shop without the requisite skills to deal with a brittle stone and complex template.

Even a great fabricator and structurally solid stone would have issues with that design.

Unfortunately this isn't the underside ... It's the top of the counter on the outside perimeter of the peninsula. So it's very visible. And yes the pieces DON'T match! There is a creamy yellow running through the slab and the piece put it the middle is much brighter and has no yellow. I could possibly look past the seams but not the discoloration. I'm guessing they broke the middle piece while cutting so I don't know what kind of resolution there could be?

h4ppy5ue comment should give you hope.
Don't settle, call them every day, go there if you can until they can come up with a solution. If you paid by credit card, then call the credit card company. They are too many things wrong. Countertops are not cheap! Plus you are going to be mad every time you sit at that countertop. Every one sitting at that place will ask: what happened? Instead of: Wow great countertop.
What was their plan about that post in the middle of the island during templating?
You should google countertop seams pictures to see what it is supposed to look like.
Don't give up.

Thank you! The post is a support beam and unfortunately there's no way around it! We removed a wall. They are going to wrap the beam and trim it so at least that part will improve.