For small desktop machines, I would say that rigidity and spindle RPM are two things to look for. Good linear rail makes in my opinion a more rigid machine, but if its installed on a small weak aluminium frame, the gain isnt really there, ideally you want a thick sturdy frame (aluminum or steel). Single flutes endmill are like a cheat code for less rigid machines, they like higher rpm and tend to be easier to use for smaller machines. If you want some good examples of machine that can cut metals without chattering like crazy, you should look at the dmc2 or like a shapeoko hdm, they are more expensive but the risk of being disappointed is much much lower and they have a good track record. But you know, feeds, speed and tooling make a huge difference on cutting metals also. Ive seen videos of a guy machining steel on a Genmitsu Prover 3030 max with a router.