I have not had any trouble with my 4n1 but I do see that there are a couple of vulnerablities.
First is that you really don't want to clamp down hard on a solid non crushable object in the middle of the bucket. Imagine using it as a fence post puller and clamping hard on a vertical metal tube filled with concrete. The middle of the movable jaw is not well supported for that sort of pressure and the combination of 4n1 hydraulics plus the power of the FEL could combine to injure (bend) the movable jaw.
The second concern I have is that on some designs (like my WRLong) the grease zerks can be found in pretty dumb places so that they are constantly jammed up with dirt. It makes greasing a hassle and raises the possibility that grit is being forced into the zerk. That may not be an issue for all designs.
The final concern I have is not so much for the 4n1 bucket but rather for the FEL arms when using the 4n1 in bulldozer mode. Tractor FEL arms are long and vulnerable compared to true bulldozer arms and it is certainly possible to hurt the FEL by hitting an immovable object off center if you are moving with a lot of power or speed.
I like my 4n1 but like most "swiss army knife" tools, it performs none of its advertized functions as well or as safely as a dedicated tool so use it cautiously. I use mine as an all around bucket when I don't have a very specific task in mind. I switch to the
grapple immediately if I think I'm going to be moving more than an occasional piece of wood or brush or larger stump. I can easily get 5 times the amount of brush in a
grapple and the load doesn't fall out while moving. The 4n1 has less bucket capacity so is not the preferred tool for moving big volumes of dirt or gravel either. I like it a lot for helping to clean up a bit after I've used the backhoe to take out a stump and it is useful as a bulldozer blade for small brush or moving dirt back into a hole. I don't dig with it much (I have a BH and don't have a toothbar). I have not done much backdragging/cutting with the jaw open (the 4th function).