In case anyone mistakes what I have said, by mini-5th I mean the type that flags the kite on one front line, running from the end of the front line at a swivel or fitting above the bar, down through the bar and CL assembly allowing a spinnable safety.
That other thing, that effectively just pulls in a lot of both front lines, is just stupid, whether it is called mini-5th also or anything else.
When it comes to flagging the kite, ideally you should be able to flag it on any line attached to it.
So a 5 line kite should be killable by releasing it to ANY of the five lines attached.
Sadly, kites aren't usually set up that way.
I came close when i added 5th line to a 2009 slingshot fuel.
I had primary safety via the 5th line, with a pigtail on the end of one front line at the fitting atop the depower rope as an alternate (non-spinnable) leash attachment point, and the 2 OSH handles.
Out of 5 lines, 4 would allow throwing the bar and killing the kite.
If I didn't want 5th for a session, for example in light wind with waves, I just attached the leash to the pigtail on the front line or swapped the pigtail for the CSS (mini 5th) safety line.
With the CSS/mini 5th I had a spinnable leash and still 2 OSH backup options.
If I just leashed onto the pigtail without the line through the bar, I had a good but non-spinnable front line safety.
Good to have options!
If you have experience and luck, you may never have had your primary safety fail. Yet.
But eventually it will happen. A friend had a problem recently where something went wrong with his safety and he had no idea what to do, and no secondary safety. Happily he was able to release his leash, but he got wrapped in lines swimming to his kite.
Luckily he got picked up.
But how would you like to get picked up by a boat, while wrapped in your lines, with the kite not completely flagged out?
Could have been a disaster!
His lines were so knotted up, still not sure what happened, and no idea or plan B (except releasing the kite) for his safety not working.
Have options, at least have a plan B, C, D, so you can TRY to use them before you get wrapped, deathlooped, dragged out to sea, or into an obstacle.
If nothing else it might save you a line set or a kite repair...