Without doubt the question of rigidity of the vertical slide on the ML10 versus the WM14 is paramount.
You cannot expect the vertical slide to be as rigid as the Mill ! Fact. Even small cuts with small endmills cause deviation and chatter and the matter of squareness is there as well. With this is the fact that the highest speed (Rpm) on the ML10 is about 820.
The 'Envelope' of the operating range of the Vertical slide is rather restricted and you can run out of room very quickly whereas the mill will have approx 150 x 500 with greater head depth.
Lots of people use the Vertical slide quite happily as they have no other recourse having only the one machine. But, it maybe OK for boreing a larger hole in a part or minor slotting but nothing big.
Clive