This is a very good question and simple to answer.
The ones I would consider worth purchasing are too expensive and weigh a ton as they are built from steel. They use linear rails and ball screws or rack and pinon for the transmission and movement. They use quality electronics with proper cooling and protection. One problem is they are so over-built for what is needed for the professional hobbyist’s and cost what a new truck would be.
The China built ones with the extruded aluminum as the frame with steel plates, bolts, nuts, washers, shims, v-groove bearings, skate bearings or just wheels for movement, takes days to build and are always needing adjustments, tightening and calibration all the time. Then they use lead-screws or belts for there transmission.
Gatton CNC & Bobs CNC are two wood builds.
Gatton provides a parts kit for most the wood pieces to complete the build. You will need to determine what size your X & Y will be and build them. Then you have to order all the other components needed to complete the build. Problem you will need to order from a “sleuth” of companies/manufactures the items needed to build and remember to adjust for your determined X & Y size.
Bobs CNC are complete kits. They have everything needed to build the CNC. I really like his design and it implements a “minimalist” engineered design and looks cool. Problem is that is has too many pieces, screws, nuts, washers and bearings to build and takes days to build.
Both Gatton and Bobs CNC use inferior transmission and movement components. This is one of the most costly and important features that is needed for a quality CNC machine.
Now we have the electronics. They all miss. This is one area that most kits “cheap out on”. You have to match your steppers to the size and weight of the axis. Then match the steppers to the correct drivers. Determine what controller to use. I use GRBL and most the others do as well. Now we determine how to cool the electronics and protect them from the dust (Most do not). Then we need to match a power supply to all the components needing power and proper routing of the wires to not interfere with the movement of the CNC. This is just the brief overview of what should be considered.
All the machines above have a place, just not for me. This is why I decided to use what the industrial machines are using for their transmission and movements. I combined this with the correct electronics, cooling, protection for the electronics and frame/structure for the Harbour CNC.