It's not my favorite look, but whether it works depends a lot on the material choice for the window return. For example, your first picture looks like a quartz slab backsplash and a drywall return at the window. It's jarring, and it looks cheap and unfinished. The second picture works because the painted drywall wraps from wall to return, so it's a cohesive, clean look. I think you need the same material for wall and return, or you need casing and trim.

In a transitional house, I'd do all windows will sills.

Can you explain what you mean by drywall return at window? I can’t follow exactly what you mean and how it’s different in both pictures?

Sure. Looking at the first picture, there's the wall with the patterned quartz. Then at the window, there's a 90-degree turn to another surface. Horizontal surface at the bottom of the window (where you would have a sill), vertical runs along the sides of the window, another horizontal surface at the top of the window. Those are the returns. In that first picture, you can only see right vertical return. But it looks like painted drywall, and it's jarring.

I'm not a pro, so it's entirely possible that I'm using the wrong term. Basically, the area where you would have casing, if you were doing picture-frame trim.

The only time I did something like that was in a shower with a vinyl window. All waterproofed and tiled right up to the window.

Previous house had bullnose drywall return with a wood stool and apron. Hated the drywall returns. Not using trim is mostly done to save money.

We built last year and I went with real trim around all my windows and doors.

One concern with wrapping a window with a slab product would be if you ever have a problem and need to replace the window.

This one looks like it has a narrow simple trim. Looks more finished to me.

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The photos make me think of a mausoleum. I don't have anything against them but I wouldn't want to prepare dinner in one.

I would not really want a regular wood sill or trim plonked on top of a stone slab.

"Mausoleum" I think that's it.

Yes, please do not do anything unusual that makes you happy. A bunch of strangers on the internet will worry about whether your house has resale value to someone you have not met yet.

I am not a stranger, I am your friend.

If the OP had been really happy with what their designer had recommended they would not have posted the question here. Designers are always happy to try different things in other people's homes! They can add an extra photo to their portfolio and they don't have to live with the results! I think OP is looking for validation to stand up to the designer ... just the vibe I got from the question.

Except . . . that the question asked about

"having a kitchen window without casing?"

not about whether to use a slab style backsplash at all--I don't see where the OP is questioning the actual use of slabs for the backsplash in the least, only about how to detail it around a window.

I was only talking about the window casing choice and not the slab ... others here seem stuck on the slab part!

"My designer suggested for any areas where there is tile/slab (kitchen and bathrooms), basically make the tile go until the window and don't add any casing."

I think it could be nice looking but would have to be finished really well so you didn’t see any weird edges on the inside of the window. We considered finishing ours off with having the tile turn back to the window frame, but we ultimately decided to do a small wood trim, because I didn’t want to have to deal with drilling into tile if I ever wanted to change the window treatments down the road.

I wouldn’t do trim on top of the slab. Too many finishes with the slab, window and then wood.