I agree that flex arms are not useful for precise positioning. At most, I find them useful to position largish diffusers and reflectors when a precise positioning is not required.
For precise positioning, it is hard to beat an XYZ micrometric stage, or a microscope stand and fine focuser for even more precise Z adjustments. There are also micrometric tilt stages (usually available in sets of two for XY tilting about the same center) that can be stacked on top of translation stages. A stack of two linear stages for XY translation, one Z stage for focusing, and two tilt stages for XY rotation (up to roughly 20°) can be 10-20 cm tall and you must account for this when building a stand. The micrometric stages most commonly used in photomacrography are probably those with a 60 by 60 mm base. There are also stages with a 40 by 40 mm base, but their micrometers are really tiny to operate, and close to each other.
If you do need an arm for precise positioning, you should not use arms that have matching rosettes to lock the arm's "elbow". These rosettes ensure that there is no slippage, but force the arm to be locked at discrete positions (usually every 2° or 4° angles, or multiples thereof). The elbow cannot lock at intermediate angles. Friction arms can be freely positioned, but must be locked tightly to prevent slippage, which makes their use impractical above 2x or so.
The Manfrotto mini hydrostatic arms can be freely positioned, and locked tightly by using a very low force. This reduces the risk of accidentally moving the arm while tightening it, and eliminates the risk of slippage. The arms are also stiff enough that flexing is not a problem for moderate loads. These arms are available in two or three different sizes. Note that these are not the same as friction arms, and that they are quite more expensive than the latter. Even a hydrostatic arm does not allow an equally precise positioning as a stack of micrometric stages, but usually works well enough to position a subject for up to roughly 5x magnification.