Is it an island or peninsula? If peninsula, isn't it attached to something already? Do you need the uppers? Or can you put stuff in the uppers that you hardly ever use? There are big wheel casters with lock out there but it would depend on what you are working with cabinet-wise.

Somebody will know the specific issues with this, I'm sure.

I think if you want to make it detachable, it stops being a peninsula and becomes a movable island that you just keep in that peninsula-like spot.
I think it can look great as long as it looks like an intensional design element and not an afterthought. I probably would do a different countertop material, each countertop would have a finished edge, I might make one slightly lower or higher, probably the island lower so it can slide under the other countertop edge.
You can make sure it doesn't roll with locking wheels, these have a heavy look though and may not work in your design style. It will be heavy and depending on how many drawers, what's in them, it may be unwieldy(think full cart at BJ's, Costco, Sam's Club).
Different surfaces

[Eclectic Kitchen design[(https://www.houzz.com/photos/eclectic-kitchen-ideas-phbr1-bp~t_709~s_2104) by Chicago Kitchen And Bath Rebekah Zaveloff
Half of this island is moveable, with split in countertop:

[Modern Kitchen design[(https://www.houzz.com/photos/modern-kitchen-ideas-phbr1-bp~t_709~s_2105) by Austin Architect Hugh Jefferson Randolph Architects
If you visualize this island pushed up against the hutch, is that what you want?

[Eclectic Kitchen design[(https://www.houzz.com/photos/eclectic-kitchen-ideas-phbr1-bp~t_709~s_2104) by Seattle Photographer Louise Lakier

Oh, wow, thanks, those pictures are so helpful.
You are right, there's no way to make it really look "flush", but maybe a tiny island that's always pushed in to make it like a peninsula? Probably will make me crazy, with crumbs falling in the crevice...