On the other hand, if variable auto-transformers and isolated fixed transformers are both off-the-shelf products, then you don't need to pay tooling charges.
There could also be a problem with making a low voltage variac. There can always be at least one shorted turn where the wiper moves from one secondary winding to another. If the secondary windings are large, that would cause a large short-circuit current. Also the adjustment step would be a large fraction of the voltage.
In the case of the 5 V, 200 A power supply that I used, I guess the transformer would give 1 V per turn, maybe more. So a variac would end up as a 0 - 1 - 2 - 3 - 4 - 5 V switch, which gives coarse adjustment. If a 240 V variable auto transformer is used, followed by a fixed 240 - 5 V transformer, the variac would also be about 1 V per turn, but that would only be 1/240*5 = 0.02 V, so the adjustment is much finer.