Direct drive works fine, if the servo is big enough.
About 400W, 1.3 Nm continuous, and == 3.9 Nm peak, and up, should do ok on linear guide machines.
Because of the high peak torque, it should/might also be fine on guideways as long as there is no binding.
For comparison, 3.9Nm is what you can twist with your hand, but is fairly hard to do.
Clearly harder than normal machine handles are turned.
I use much bigger servos, and I gear them down 1:2.
I do so to get more resolution and accuracy.
10.000 count servos, 750W, 220V, at 1:2, with HTD8-30 mm wide belts.
I don´t need all the accuracy, nor the top speed, but the extra resolution is desirable to me.
Step size is 0.2 um.
Step size is =/= actual machine resolution or repeatability.
Actual incremental move size is less than one micron with 4 mm screws (32 mm D).
Lathe.
The smaller servos mentioned, 400W, I use on a VMC.
At 1:3, HTD 5-15 mm.
5000 count servos, 60V.
A mill does not need anywhere the resolution that a lathe does.
Industrial machines are mostly direct drive these days ..
but they use 1 kW (older) to 2 kW (newer) servos.
About 10.000 counts (Haas, since ==2012), older Haas was 2000 count.
Industrially they are looking for "speed" in acceleration and top speed .. totally not needed for non-production machines.
And the tables are easily 1000 kg or more in mass.
My VMC table is 200 kg now, and 400 kg+ soon.
For calcs:
3 Nm at 1:3 == approx. 600 kgf push force on the table.
Example for bigger servo, 750W.
The torque is 10 Nm (peak at servo) x 2 = 20 Nm.
Push force is T = Fl / 2 x pi x efficiency
T - torque in Nm, 20 Nm
F = push force in N
l = screw lead in m, 0.004 m
efficiency 96% (ballscrew)
F = 2 x 3.14 x 0.96 / 0.004
= 15072 N = 1500 kgf.
A 32 mm ballscrew is rated at about 1400-1500 kgf push force.
This is std on industrial machines, medium size VMCs, and is what I used for myself.