I don't see a lot of cons for the knaider. It's the swaider that increases the risk factor for me.
1) encourages longer lineman belt usage, which decreases effectiveness of said LB.
2) its longer length leads to bigger foot swing in a kickout situation. Granted, most don't experience these, but when they happen, they suck bad. Ask me how I know.
3) Mechanically, there's a lot of lower body adjustment (hips, knees, e.t.c) taking place as you transfer your weight from one step to another. Using a double aider system means that this adjustment takes place on a wobbly system. A single aider system has alternating firm footing (stick or step) so that lower body adjustment has less opportunity to cause a problem. Just my 2 cents here. Not sure if this makes sense.
Above all, climbing is dangerous and I'm not a climbing expert. I'm just some dude on the internet.