For roots you need something shorter with more leverage. The
M59 has a powerful loader - lifts 4000 lbs to full height and is stable. But even its BH bucket can be stalled by a few small roots. The rock bucket is long enough to be stalled by a backberry thicket.
The trick to using the rock bucket is to first use it as a fork to loosen the soil, then just push the rock bucket into a loosened soil sort about a foot, then lift it and tilt bringing up soil and rocks and hoping the soil fall through as the rocks roll back.
Soil plus roots won't fall through. But the rock bucket is good for forking the soil - but then so are some of the other grapple buckets and they are probably better for ripping the soil even if they don't carry anything.
For branches or trash the rock bucket is great. It sticks out the right amount to hold a huge load, has tie downs everywhere, and can spear things.