Got to throw in with the other side of the coin. A SB, which is a fabulous machine for the money, is not a super high resolution big iron machine. If you go into it with the appropriate expectations then you will be amazed. If you expect unreasonable perfection then you have only yourself to blame for your over inflated expectations and resulting disappointment.
I could make parts that competed with the big boys on quality any time, but they did it fast and easy and for us it took a lot of tweaking, patience and time. Brady has gotten more out of his mills than darn near anyone could. He is also the first to admit the immense amount of time and effort in mechanical education it has taken to do that. Nothing wrong with that at all. Fabulous in fact. But, that is also far beyond what most mill clients want to have to do. Note that when it comes to flatbed mills they all end up in the same place though, There is a spoil board thrown on top of the plenum. The spoil board changes shape during it's life. That affects part production. Learning how, when, and what to do about it is what separates the Pros from the Ams. No matter how flat, stiff, and non reactive your base below it is, the spoil board above and the live part you are cutting are still the final variables to deal with. To do that we don't need to be Scientists. Artisanship is plenty sufficient.