the real deal
Spearguns are only and always just platforms for the shaft to be propelled by the bands that stretch the length of the stock/barrel. Thus, the longer the gun, the more the bands are stretched and the more power available to the shaft.
Thing is that some conditions and fish don't let you use a 6 foot gun, but you still need the power to sling the shaft. That's where a mid handle comes into play. By having the handle in the back tird or so of the gun, you get a couple of things- The handle is in the "middle" so you can swing the gun around easier. This is nice when in the rocks, the kelp and when you have a real log of a gun, like my Alexander, my Merlo and my own guns. A mid handle also positions the trigger mechanism at the very back of the stock, giving you that length from the handle to the trigger mechanism as additional stretch to the bands, in the case of my guns that adds up to 18". A mid handle also balances better.
If you have a handle/trigger at the end of the gun,as in a euro or Riffe Competitor, or a JBL you often have a real bear loading it as the notches/tabs are waaay back there against your gut. A handle at the end is also a very great pain in the ass in the kelp, the current and in the rocks. It is a great way to go out in the open reefs like Hawaii and Florida where the added length you gain from having all the gun out in front of you puts the tip that much closer to the fish.
Picasso is a euro gun, as are OMER's, Rob Allens and Esclapez and they use a trigger totally different from a Riffe or Alexander- the US lumber style guns. The euros act to pinch the shaft rather than having a definite dog or sear to hold the shaft. They can, as can any gun be overpowered and suffer a misfire, but that's pretty rare and then most often happens with a cluster of mistakes. Euro's are sometimes thought to be faster as there is less gun in terms of it's mass, but in reality, you're as fast as you make yourself by anticipating the prey and positioning your body as you go along. Again, it depends on what you get used to, what your prey and water is, and your skill levels. After that it's all about wishing you had a different gun for that special trip.