I think the entirety of the A-series, M-series and even S-series lines are essentially one chip product line, with different balances of chip area, cost, compute and energy use.

Other than that, perhaps some small form factor related device support differences.

Never been an OS (iOS, iPad, watchOS vs. Mac) distinction from the hardware standpoint.

The only thing I read from M-series in iPads and A-series in the Neo, is the A chip is better balanced in price and power draw for a low cost laptop with a smaller battery.

The M-chip with that balance is the A-chip.

I don't think they would; I'm sure they share a lot of the low level code already, the main difference now is in the user interface and software.

Some time ago (...over ten years ago) they made some movements towards unifying the desktop and tablet interfaces with LaunchPad, which looked like it was designed for a touch screen, but they never followed through. Not even touch screens on their laptops, which honestly still surprises me.

Shipping macOS on iPads will near instantly vanish a very large cohort of their macbook users.

I think it's purely a pricing & supply chain thing. Certain iPads have M-series chips in them, now certain MacBooks have A-series chips in them.

Also, the chip used has no impact on the viability of merging macOS and iOS anyway.

Haven't they? I can download iOS apps from the app store, sign them again with my own keys for MacOS, run them natively on my MacBook without any issues. Same binaries, same APIs. It all just works.

Seeing as it's the A18, are there any concerns about third-party "desktop" software not running on this new platform?

Why would that mean that, when iPads have had A series and M series chips for ages now?

Hasn’t that been the discussion since the laptops/desktops went arm?

in a competitive market they would have been unified a long time ago. google has been making slow steps at doing this apple wont until google does