Assisted opening.

[evilgreg]

More to break.

Increases cost.

Doesn't help people not see your knife as a weapon and unless you have some kind of disability they are completely needed.

And finally... closing the knife. Why the hell would I want a spring on an axis lock knife that keeps me from closing it with the axis flip?

+1 to this. I was going to write a reply, but I see that Styg has already covered my main objections (annoying to close, extra junk to break, etc.) The other complaint I have is that assist mechanisms are wholly unnecessary. We have the technology to make great unassisted flippers without the baggage, and I can flick open my non-flipper knives (e.g. Sage 2, BM 940, BM 710) just as quick anyway.

In short, assist mechanisms are the appendix of folding pocket knives to me. Does nothing useful, and might hose you up. And before you say "just take out the spring", that's great for knives with a detent of some type, but doesn't work at all for others (try it on a Kershaw Cryo, for example, and see how that works out when it's flopping open in your pocket).