In terms of accuracy, there's no beating the big industrial machines. Röders, Willemin-Macodel, DMG, Datron, etc. They all have dental specific applications. They ain't cheap but you get what you pay for. There's a reason all of these machines are huge, its hard to stay stable and repeatable without mass and rigidity.
Besides being big, to obtain accuracy in the single digit micron range you need things like temperature control, laser measuring tools to detect machining surfaces, tool wobble, etc
Most medium/smaller dental machines don't have the real estate (or budget) for these things so they use other tools to get 'close'.
Things like touch probes are only looking at the tool in one dimension.
Temperature measuring (but no active temperature management system)
Tools; all tool makers have an internal tolerance of deviation between batches, one major tool maker has a tolerance of 20 microns for example, another one I know is 5.
CNC controller. This is the brains of the mill, some milling machine makers choose a 'cheaper' controller to reduce the end price of the mill, this also has a major effect on the machining and part tolerance.
So if you're going to make compromises, you should go into it with your eyes open and not believe the propaganda of the sales people. Every one of them will say their machine is the best... only one of them is telling the truth, the rest are liars... do your research and don't drink the koolaid is my advice.