Not me who originally raised the point, but I continually find that my error rate on the chicklet style keyboard is much higher than on the older style keyboards (I use a chicklet keyboard on my work supplied Thinkpad, and the old style on my personal T400, which is still going strong running Ubuntu).
If I had to say what the difference is, I would say that it is the fact that the older pattern of keys had a more pronounced edge, and a taper to actually reduce the effective size of the key, so that you could feel how close your finger was to the center, and thus where your fingers are relative to the keyboard in general. With the mostly flat key caps of the chicklet type, I do not get that feedback, and wander to the edge of the keys, and can end up pressing two keys. I can feel that, but the fact that I then have to go back and correct the mistake slows down my typing.
I'm sure that this is an artifact of the way I type, but after using computer keyboards for 40+ years (almost exclusively on keyboards with curved tops and pronounced edges, some even more so than Thinkpad keyboards), I doubt that I am likely to change my typing style.
I remember many old keyboards (Dec VT100 and VT220, IBM 3278 etc.) whose keys were significantly dished, giving very positive location feedback. Flat keys of the type I first saw on Apple systems are just wrong.