marcojohnson: Your corner can be made to look much nicer with a little effort and expertise. All solid surface joints, and yes, quartz is considered a solid surface by industry standards, are to be considered "inconspicuous", not "invisible". Yours look very nice. Methelmethacrylate seams shrink as they cure, that's why they're overfilled and scraped to flush after curing. If you scrape off the squeeze-out while it's wet, it will shrink below flush, creating a more noticeable seam. I'd hit it with some cyanoacrylate, catalyze, and scrape it and see how it looks and feels. I'm not aware of any quartz manufacturer that recommends top seam polishing and I wouldn't recommend it here. The technology just isn't here yet and I've bought and tried almost every system and taken the classes. Fabricators should retard catalyst on white seams. Adhesive manufacturers and fabricators like consistent set up times, but if the adhesive is mixed in a baggie instead of being shot through a mixer tip, a third of the catalyst can be removed. The seam takes longer to set up, but it is much less likely to yellow. If your seam yellows, it probably won't show up for at least a month or so. You can see a picture of a yellowed white seam on my IKEA Tampa thread. It is very difficult to evaluate seam quality from laptop pictures.

My counter seam with a fine-grained quartz is a hair's width. Part of the reason is that they used a machine that pulls the slabs together. Not sure that's possible with a backsplash. Are you doing a full quartz backsplash? Some people mistakenly do 4" of the counter material and a tiled backsplash above that. You'd want to avoid that.

Your seam can be the width of a hair and almost invisible with quartz. You shouldn't have chippy edges like you might with a granite with crystals, especially if your quartz is marble-like or other quiet pattern that is basically resin and fine sand particles. They can also use a seam setter that electronically pulls the two slabs together. That's what I attribute my very tight seam to, but I'm not sure if this is an option with a sink seam where there's less of the slab to grip. The colorant is the least of your worries as that can be adjusted. Ask to see samples of seams they've done. Also ask to see the slab and how the template will be placed. Quartz can have unsightly resin pools (voids where there's no quartz pieces) that you'll want to template around. Cambria seems to be especially prone to resin pooling. Also your slab may be very different than the sample.

The dip is caused by fabricator/installer error. It is not from over polishing. I'm betting the original seam edges were cut out of square which made them have to shim the short ends of the top up to get the seam to close. The shop caused the installer to choose between a tight seam and a dip or a poor seam that was flat. I'd put a fresh diamond blade in a Festool rail saw and cut the seam loose in three passes. (The heater bar trick works for stone, but it may discolor estone; too much risk.) Dust free with a shop vac on the front of the Festool. Then I'd dress the freshly cut edges with a Seam Phantom on site, again dust free. I'd shim the cabinets to eliminate the dip and pull the dressed edges together with Gorilla Grips. May have to borrow another pair depending on the island width. I'd scrape the seam with a porcelain blade to avoid metal marks on that light quartz. Polish the edges to match the top. No cabinet damage, but the island will be 1/8" shorter than before. One day. Maybe two guys depending on the island size. Make 'em fix it please. You're doing them a favor in the long run.