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From my experience, the traces on the later fiberglass boards were much more susceptible to lifting from the board while soldering and working on them. The traces are not easy to fix or replace once broken either. However, the fiberglass boards did not tend to develop that waxy or gooey film on them, which the older boards seemed to do. I'm not sure what caused that.
Dynaco did make an effort to upgrade the cheap brown phenolic board on the ST-70 in the early 1970's by replacing the "brown board" with a much more durable and heat resistant pale yellow fiberglass board. It still had open solder traces but for it's time it was much more durable and a step up from the original brown board. Photos below.
Bob
Earlier phenolic board top
Earlier phenolic board bottom
Later fiberglass board top
Later fiberglass board bottom