Just a view thoughts on this;
- More copper means thinner prepeg layers. The resin in the prepeg layers will fill the gaps between the copper during lamination. More copper means less space to fill and thinner prepeg layers can be used.
- More copper will reduce warpage of the board.
- more copper is also a big advantage at the plating process. More copper will concentrate more field lines during the plating proces. The plating proces can be much better controlled.
- more copper means less copper to etch away. this will reduce the amount of under-etching.