There is plenty wrong with the basic machine design.
If you're only doing softer materials with light cuts, it's not a problem, but if you try pushing the spindle, you'll get resonance/poor cuts.

If you have a DTI, try setting it up against the spindle noseat a typical cutting height, then apply some pressure against the spindle and see how much deflection you get. Having three sets of unsupported rails between the bed and the spindle does not produce good rigidity.
Then try testing for backlash. The setup used for the ballscrew endfloat is a couple single row ball bearings (which are usually cheap rubbish), with preload controlled via a single nyloc nut. Then the stepper motor couplers will most likely be cheap aluminium bellow type, which will crack if driven hard.

Plus the general tolerances will be questionable.

And in case you're wondering why I know all this, is I bought a 3040z as the base for a digitizing machine, knowing full well it's shortcomings. It does it's job as digitiser, but I certainly wouldn't want to use it as a router and expect it to produce good quality cuts and not need continual fettling.