Imagine your cellphone. Now imagine it the size of a smart watch, yet with no loss of ability. Picture also the size of the cooling capacity also similarly shrunk. That's taking regular atx and stuffing it all in a sff case. Now imagine the cooling potential if you stuck all that in something the size of a laptop. You'd never overheat the cellphone. That's taking sff and sticking it in a full tower.
The smaller the case, the more important cooling ability and potential becomes. Corsair did a really good job with the One, but it's still limited in a physical way, such as a proprietary 180mm AIO, limited airflow on the gpu etc.
For almost all mITX/sff builds, Ryzen is King. Intel top line cpus are simply far too high wattage to allow for sufficient cooling. Taking something like a 9900k - 12900k and trying to cool it with a Noctua L9i that's barely 120w capable is cpu suicide.
The One? It's decent. Does just fine for what it is, but for a heavy gaming or hard line production pc, it's next to useless. It has far too limited range of thermals, you'll be knee deep in a raid or halfway through a render and suddenly notice fps has tanked, the render now takes 4x as long as when you started etc, just because the AIO is heat soaked.
If you are serious about mitx/sff, you'll need to start from scratch, throw out anything you thought you knew, and do a ton of research on airflow, thermals, capacity, wattages, dimensions. My pc took a year to plan and build, down to mm clearances on parts, that later had to be scratched and started over when another part was mm to large.
ATX builders only get concerned with height of the air cooler and ram, mITX apples that concern to every single component, most times being limited on choice of exactly 1 brand and model of something, because it's the only thing that fits.