Riding with no hands
 

Why is it easier for me to steer my old comfort bike with no hands, but my road bike and my mountain bike are much more unstable.

Probably because you're not really comfortable with the handling yet, or the dropouts are possibly out of alignment. When I first got my 5200 last year, the steering felt much quicker than my 2100, and appeared to be harder to ride with no hands. Now, it feels like anything else.

Comfort bike probably has a longer wheel base. Now that I am riding a bike with a shorter top tube, I can barely ride with no hands because it's squirrely. And it looks like it's going to get shorter since I toasted the fork and can only find one with a slightly smaller rake.

I think that you also have a higher center of gravity on your roadie. That definitely affects stability.

Make sure that you're sitting straight up, not leaning forward. Get your weight over the rear tire. Keep pedaling in a gear that has some tension to it.

Comfort bike has more relaxed geometry, it's inherently more stable.

All the above is true. Deep dish wheels can affect it too. Riding Zipp 404's no handed with a cross wind canbe alittle sketchy sometimes.

I cannot ride my Cannondale handless for the life of me. The first two seconds is a little scary, but then on the slightest imperfection in the road makes the bike automatically lurch in the most dangerous direction.

No hands at 30mph is a heck of alot easier than no hands a 10mph.

I only ride no hands at safe speeds, 20Mph+.