No matter how good a particular reel is and how experienced the diver, the reel inherently comes with the potential to jam due to the line getting entangled in it or with another speargun component. Apart from some really freakish accident where the reel line can actually entangle the diver and not allow him to surface, there is a much more common way for a reel to kill a diver. Imagine this common scenario; an expensive $500 speargun, with a $150 reel and line, and a $500 GoPro/LCD bacpac/mount attached (total value well over $1000), diving 60ft (serious stuff for most of us) in bad vis with a stiff current over a reef with no prominent features. Diver is near the end of his breath hold stalking a fish, he shoots and hits it. Either the fish is big and strong and puts up a fight, or it holes up, or the shaft sticks in the bottom, and then the reel jams. At this point the diver really needs to surface, but he doesn't want to leave the gun. Letting go in these conditions means saying goodbye to $1000 worth of gear. But that's not a safety issue. The safety issue that most of us will take an extra few seconds to try and free the reel, or take out a knife and cut the line before making that decision, and by the time we do a blackout may be unavoidable. I know this because I was there when it happened to someone.
Another dangerous aspect of the reel is that it not only puts the user at risk, but his dive buddies too. Another common scenario; a boat driver with 4 divers in the water with the divers spread out over a large area. The sea becomes a little choppy, and even floats without flags become difficult to spot. The diver with a reel and no float line/float is impossible for the driver to see until he dives and throws a leg up, or raises his gun out of the water. Suddenly one of the divers starts hollering at the boat to come over. It could be any one of serious problems; shark, blackout, entanglement, all the driver know is that he better get over there fast. But he can't until he knows exactly where each of the other 3 divers is, so as not to run them over on the way to the diver in need of help. This will cause a delay, possibly with serious consequences.
For this reason on my boat everyone must have a float line, float and flag. I've seen guys try to get around this rule and use a reel saying that they'll stay near a diver that has a float. I find that inevitably they separate creating the same problem. Also I find them consistently selfish as they will shoot fish and instead of swimming them to the boat will clip them to the stringer on their buddy's float, increasing the drag and making the buddy work harder while they enjoy "the freedom" of the reel. For this reason another one of my rules is; all fish on my stringer are mine, if someone puts their fish on my stringer the fish now belongs to me.
A float line by contrast is worry free. It's easy for the boat driver to know exactly where the divers are and to go collect them when they unknowingly ranged too far from the others, before they too disappear from sight even with the float and flag. No matter what happens I'm never concerned about losing my gear because there's a float at the end of the line. That's why it's called a float line, although some will argue it's because the line itself floats. In any case together with a highly visible dive flag the float will always allow me to find my gear. So if I feel I'm in trouble, all I do is let go of everything and surface. Actually it's a reflex for me to let go of the gun right after the shot (if I hit the fish that is) and after reach for the float line and play the fish or surface straight away if I feel I have to. So, If a float line is so safe then what are diver's reasons against it and for using a reel?
BTW this is a good Spearfishing float line