No negative effects electrically.
Practically, it takes considerably more heat to solder 12 gauge wire to rail, which will significantly increase your risk of melted ties. Also, the solid 12 gauge is not flexible so may cause misalignments in your track when you pull the feeder through the holes in the benchwork. Last disadvantage to such heavy feeders is hiding them visually.
Personally, I go the opposite direction using 26 gauge solid magnet wire for feeders. Magnet wire has a varnish coating for insulation. The feeders are soldered to the underside of the rail where practical, on the side where it's not. The feeder is tied to the bus which is much heavier (16 gauge) underneath the layout. My feeders are never more than 12 inches long and often less.
I don't run long trains on a small layout, and I at most will double-head 2 locos, so my current draws are modest (less than 1 amp) even with older locomotives with open frame motors.
my thoughts, your choices