It's difficult to compare wooden machines with aluminum because the materials are very different. It is a general conclusion that the machine will not cut reliably any materials that are harder than ones it is built from. So a wooden machine will not be able to cut plastics and metals with good quality.
I have no experience with wooden machines, but I'm pretty confident they won't be very precise. My guess is 1mm - 0.5mm at best. For cutting wood that is enough though.
With aluminum based machine you can get tolerances up to 0.1mm or sometimes even better. With steel (and good design) - 0.05mm or even 0.01mm.
As you see, this really depends on what you need and how much you can spend (both money and time) on your machine.
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