I have a Proxxon MF70 that I am converting it to CNC. My expectations ... based on my manual use of the mill ... are wood, plastic and aluminum with a great deal of patience as incredibly low feed rates will be necessary, and rational thought related to its significant size and rigidity constraints being applied. Mild steel is, IMHO, a non-starter. However, I want to experiment with CNC and using relatively easy to acquire steppers and mounts, the Protoneer Hat on top of a Raspberry Pi and the ubiquitous Pololu Drivers I am on the way to learning CNC and expected "breaking stuff" without having to spend a lot of money.
I can not comment on the SX1 or the Taig but do have the Harbor Freight clone of the SX2, again not CNC but used manually. That is suitable, with the correct application of common sense and patience, for aluminum and mild steel.
Just my thoughts,
Arvid