I use the tie fast tool too and like it. The "double nail knot" seems to me to be closer to a double uni knot.

It seems to hold fine for me.

I use either a blood knot or the knot with the tie fast tool depending on where I am. I'll typically start off with a blood knot but while I'm on the stream or at the lake, I find it faster to use the tie fast tool than tying a "proper" blood knot.

Jeff