Small Form Factor uATX vs. ATX

Downsizing doesn't necessarily remove features, but will definitely remove some ports (PCI-E etc.). There's no reason a decent mATX board won't have the same feature set as the ATX equivalent as long as you're willing to potentially pay for that privilege.
Size of the board has no bearing on "speed" or compatibility, that's largely down to manufacturer support and the other kit involved. (Speed is a very general term. Identically specced boards will perform very similarly irrespective of size of board. You may have a slight latency advantage on smaller boards though which can translate to improved performance in certain workloads.)
Smaller boards can be more difficult to cool though I've never had the issue with mATX as it's still fairly large.

On a side note, probably better to start your own thread as the others have alluded to rather than resurrecting a 17 year old one

 

Downsizing removes some features, not much.

If you look at Intel B560 as example, most B560 full size ATX boards have 3 M.2 slots. But a quick search shows mATX boards only have 2 M.2 slots.

mATX also remove features in the sense that the PCIe 4x slot may not work with a large (3 slot) graphics card.

But for speed and cooling, mATX and ATX doesn't really change much. As long as the board isn't super cheap OEM (who are mATX most of the time), they all should have adequate VRM solutions (power capability and cooling) to feed power to the CPU.


I personally think it depends on your use-case. I rate M.2 slots highly because I have obtained a few spare 1TB drives. My old AMD B550 mATX only hold 2 M.2 drives natively and the 4x slot is blocked by graphics card. I upgraded to full size ATX board (AMD x570) and it has 3 M.2 slots plus I can use an adaptor to use PCIe 4x slot for more M.2 drives.

 

FWIW, there was a response to your post.

 

You would have done better if you started a new thread, rather than resurrecting a thread last responded to more than 17½ years ago.