A major down side to using gold as an etch resist is the undercut/slivers
problem.  Nickel is brittle where it overhangs the edges.

And lets don't encourage any one to go to selective electroplated gold
areas.  The problems get compounded when you do selective gold.  If part
of the board is gold as an etch resist and another part of the board is
tin or solder as an etch resist you get a galvanic effect between the
dissimilar metals in the etcher.  The copper near the tin will etch twice
as fast as the copper near the gold.  This makes it very difficult to
control line widths.  It is not uncommon to see lines at half the expected
width on part of the board and see shorts in other areas.

We sometines compensate the line widths larger in the tin part of the
board to accommodate etching and leave the gold area with no comp.
Another troublesome area is the butt line where the gold and tin come
together.  We always hope this happens in an area where it can be
covered up with soldermask because it gets ugly.

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