A few things about fluorocarbon:
Make sure you’re buying 100% fluorocarbon. Some of the stuff out there is not 100% or is just coated.
Fluorocarbon is not invisible, but unlike nylon monofilament, it has the same refractive index as water.
As noted above, one of the biggest advantages of fluorocarbon is the abrasion resistance. Fluorocarbon leader is far more abrasion resistant than braid or mono. This is a huge advantage fishing around structure like oysters, docks, vegetation, etc.
Fluorocarbon also doesn’t stretch like mono and therefore makes better leader material. When mono stretches, the diameter decreases and it inherently becomes even less abrasion resistant than mono.
Fluorocarbon sinks. It’s not the best line or leader for topwater applications.
I disagree with others about using fluorocarbon line as leader. Fluorocarbon line is much softer than the leader material and not nearly as abrasion resistant. It is made to be supple and to cast easily, not to hold up as leader.
Even though our water is muddy, my opinion is that it does make a difference, particularly in the winter when the water is clear and the fish are pickier and not as active. I’ve seen first-hand fluorocarbon outfish mono in our inshore waters. Sure, you can catch fish tying directly to braid or with mono, but I’ve never seen either of those outfish fluoro. For inshore fishing, using fluorocarbon certainly doesn’t hurt.
For applications like tuna fishing, it absolutely makes a difference. Ask anyone who has chunked for tuna in the last 10 years what they use.
Once fluorocarbon becomes nicked or abraded, it becomes more visible under water. Retie frequently.
I typically make my leaders longer than most - typically 4’ to 5’. This gives you even more abrasion resistance and allows you to retie frequently without tying on a new leader.
As someone else mentioned, the uni-to-uni is the most reliable connection. I use 3 wraps on the leader side and 10 wraps on the braid side. Tighten the knot very