My larger frame road bike (Centurion Ironman) felt a bit twitchy on fast descents and curves when I first swapped from the original 125mm stem to a 90mm. But I got accustomed to it quickly. The overall frame geometry is more oriented toward comfort and stability in long road races or time trials, so that offset the slight twitchy feeling.

My smaller frame bike (early '90s Trek 5900) has the original long Ibis titanium stem. I suspect the bike might feel a bit twitchy with a shorter stem. It feels a bit stretched out until my back, neck and shoulders are loosened up. But it feels very stable on fast descents and curves. The LBS owner remembers those early Trek carbon fiber bikes and said they were more oriented toward long one day and multi-day stage races and time trials, and some racers found them a bit unresponsive for crits on short courses with lots of tight turns. No big deal, I'm never riding crits again at my age. Time trials, maybe.

They are good. I have one.

I usually ride 56cm bikes. I bought a used 54cm Trek 520 and converted it successfully. I added a stem riser and a long steep stem with no other changes. Very comfortable. Note that the 520 is a drop bar touring bike and I'm happy with the bars about 1-2" higher than my seat.

Remember that saddle position is set relative to your crank position. It is not used to compensate for a short top tube. You can use a longer stem and some spacers to move the handlebar up and forward but very long stems can feel awkward when steering at lower speeds.

I started out with a 19.5" (50 cm) frame on a used bike that a salesman told me was "close enough". Setback seat post maxed out of the seat tube, seat set back on the rails, stem all the way up (old style quill stem) and a couple of changes of handlebars and eventually I got it to be what seemed fairly comfortable. Then I learned more about bike fit and got some advice from someone who knew bike fitting. Turned out that I needed a 21" or 22" (54-56 cm) frame depending on geometry. New bike of the proper size, saddle mid-rail on a neutral seat post, stem actually 1 cm shorter than stock with a slight rise and the bike fits me perfectly. It is far more comfortable and handles much better. Yes proper fit does make a difference over work-arounds for a too small frame.