Another weak link is the factory looped ends on fly lines, do yourselfs a favor and cut them off and use three nail knots on you loop and you won't lose fish becuse of a loop being pulled out.
I hate factory fly line loops, but I don't use nail knots for the same reason Dan doesn't use a surgeons knot: They fail simply based upon the crappy nature of how the knot secures on top of the line. For the life of me, I don't know how nail knots became popular -- they're mechanically....stupid in principle. I've had 2 fail me by stripping the fly line coating on 2 different of my lines (cold, more soft coated lines). I probably feel the same about nail knots as Dan might about the surgeons.
I use a castwell knot (sheet bend variation) and have landed some good fish on my tackle and have never had it fail. I am 100% confident in it, and I have little to no faith in nail knots. This is kind of disappointing, because I enjoy tying nail knots and think they look cool.
The only "negative" is something I actually don't consider a negative, but the knot is of course more bulky than the nail knot. That said, I don't fish ridiculously long leaders and I don't really have a need for leader knots to go through my guides when fighting a fish. Honestly, a nail knot hangs up just about the same as the castwell, so I don't think it's much of a difference anyway.
Needleknot may be better....but again, I don't care. I don't need slim knots.