I DO know that the name of Franz Bardon is well respected in occult circles.
With regards to the lethal AP thing, he may be refering to a very hard to do and dangerous form of astral projection where you project your *full self* out of the body.
The technique Bardon talks about (from your description) is a rare type practiced by some yogis etc, where you bypass the double and project fully.
The advantage is that with a proper 'full on' projection, almost identical to what happens at death, you get a stable projection with no fluctuations and are in total control, with full clarity; the resulting AP is meant to be awesome.
This is in contrast to most common forms of AP where it is an astral double that is projected, like a remote control camera if you like, only much better as it makes you feel as if you are really out there.
With common AP everything is much more unstable and hard to control, it takes years of practice, although in time the quality of projections improve.
The downside is that with common AP, there is no risk, as no matter what happens to the double, you are always safely locked inside your body.
Howevever, with 'full-on' AP you really are outside and the link with your body is very tenuous. In fact, any disturbance will break the link causing the body to die. You will be fine but you've lost your body and your current physical incarnation is now over... oops!
This rare type of AP is very difficult to achieve however and only real masters attempt it, making damn sure they are well away from anything which might disturb them; I have heard that disiples will usually stand guard around the person attempting this so that no harm will come to them; it's WAY to risky to do by yourself!
I would recomend staying well away from this type of AP and stick with normal, well known AP and phasing techniques, such as those described by Robert Bruce or Monroe.