What lb test line and leader are you tying with the FG knot?
From my experience, it is a suitable knot for upto 40-50lb braid joined to perhaps upto 60 lb Fluoro leaders, which easily covers 90% of the fishing done by most of us.
As you said, I am using leader from 40-60 pound as I don't feel the need for any heavier leaders even from jetties. Sure the heavier leader helps but they have their own consequences.
I have never tried fg knot with any heavier leaders, but I feel there should be no issues. I had seen once my friend using 130 pound leader with FG knot, no issues.
The problem arises if you go to leaders of 100lb and above, say when you are after GTs in the Andamans where they easily weigh 40-50 lbs or when you are after barra that are in dense cover and
weigh considerably more than your average barras.
Once I had handled a monster GT (it was bigger than me) with a 40 pound mono line on a penn spinfisher. I could not land it as the line got stuck on a rock just seconds before landing after a huge battle. On the other instance I did put brake on the 1 meter Barra (~13.5 Kg) without giving line (because of the proximity to rocks) on a 30 pound braid and 40 pound leader. It is really odd for me to think about using a leader greater than 60 pounds. Even with 50 pound leader I had tough time breaking the leader during the odd times when the lure got stuck in the rocks. The other reason is even the 200 pound leaders are no match for laser sharp edges of the arthropods on the rocks.