A Uni junction knot is one line wrapped around two for a total value of 3; an Alberto is one line wrapped around two, then wrapped again for a total value of 4; an FG (not an FC as the one source noted above mislabels it)is simply two lines "plaited" together for a total value of 2. So, the FG is, by far, the slimmest of these three junction knots and it tests way stronger, and is stronger, in actual applications.
But, line width is only one metric and another is the functional length of knots, so a Uni junction knot is longer than an Alberto because it comprises two knots slamming into each other while the Alberto junction knot is stacked making it shorter. That seemingly tiny extra length of thickening for the Uni makes a difference (a negative) when it is passing through the rod tip as the knot doesn't bend as well.
I can feel/hear either a Uni or an Alberto junction knot being reeled in, rarely hear or feel anything casting out line; but, an FG acts as if no knot is even there. You won't hear or feel a thing as it passes in or out.
Agreed: I only tie an FG as I put on fresh fluoro before each trip out. If I break off a leader, much harder to do with an FG, I tend to tie a Uni Junction knot on the water. I'd likely tie an Alberto but I use the Uni for my tackle knot, too, so I tie it better.
FG tip: Forget about that nonsense tying an FG by leaning the rod away to apply tension and holding the line in your teeth. This is the way I think it is best tied. It takes a bit of practice, adds a bit of time, but worth learning. Look here: Best FG tying technique