It all boils down to how much time and expertise you have! With some expertise in fabricating, electronics and computers...along with good skills locating and purchasing components you can build a pretty good cnc cutting machine.
To make a plasma torch cut with its best capability in terms of cut speed, cut quality and consumable life....the x, y and z motion capability of the machine has to be very good. If the machine has poor acceleration and deacceleration....expect wider kerf in the corners and dross. If the z axis (torch heigh control) cannot maintain the proper height under all cutting conditions to within .005" to .010".....and is not able to compensate for minor warpage in the plate.....then expect varying edge angularity and poor consumable life. If the cnc controller hiccups when the plasma torch fires, and loses position...how much metal did you just waste?
Many have built their own machines....I have done it myself. However, when I look back at the pains and the shear amount of time I spent on designing, building, and redesigning the machine....I likely was better off buying an entry level machine like the ones you mention from Samson, Torchmate and PlasmaCam.
Regardless of the reports you read....all of the manufacturers of these entry level machines have built a lot of machines...and have been through the learning curve a few times over.
Most of those that buy these machines have had some bad experiences in the first months of operation.......then after a few months things seem to start working better. It's called the learning curve in most cases! In some cases where a user is never happy with one of these machines is likely due to the fact that they expected the same performance as a $100,000 precision, high definition industrial plasma cutting machine for the price of $10,000.
I scrapped my home built machine and bought an early model 98Z PlasmaCam over 7 years ago. For the first few months I couldn't get the performance I wanted....but over time, my problems went away...and I love the machine now....and in fact am getting ready to order a brand new one with the latest features. if I had purchased the PlasmaCam instead of building a machine....I would have been through the learning curve in less time than it took to locate all the parts and get the homebuilt machine running.
Granted there is a lot of satisfaction in having a succesfull homebuilt machine. In hindsight I think that these entry level machines are a real bargain for the hobbyist...or for a small shop. Just don't expect an industrial grade machine at the hobbyist price!