Two reasons.
If it's under load, it is held shut, but when shaken, i've seen 'em come undone.
They're not as strong. A scissor hook or a snaphook is not 'closed,' meaning that one side is supported by the other, so it can open if it deforms enough.
A pin shackle, a carabiner or a snapshackle may deform, but won't release untill the material breaks.

"(PeteCresswell)" wrote in message ...
Per Jim Conlin:
I think that's called a 'scissor hook'
I would not consider it reliable.

Howcome?

Seems like with snap shackles the bail can get squinched over and
the whole thing can turn into just a hook.

The attraction of this thing for me is that once it's attached to
a steel ring and put under load, the scissoring of the
overlapping hooks seems to guarantee that it won't work loose.

All theory of course....
--
PeteCresswell