I would first attribute instability to being new to bents and not relaxing fully.
Next I would attribute some to the small front wheel.
Third is (if this hasn't changed in recent years) Lightning uses low trail geometry to minimize the influence of pedaling forces on the steering. Most modern uprights have high trail (55-65mm), where the Lightning is probably closer to 30mm. It isn't less stable, but it does take getting used to before it does not feel twitchy. My Lightning Phantom had amazingly precision handling, once I replaced the crappy stock headset.
Climbing I think suffers from not being able to use different muscles, and the mesh seat, but once you get bent specific conditioning, it is not as bad as people say, and bent conditioning does work on uprights, so going back is easy. Last time I rode a bent, I could sustain speeds of 35mph for about 5 minutes in a stretch. I haven't been able to do that on an upright for 20 years.
I no longer ride bents because newer saddle designs and big Compass tires minimized the problems I had on uprights.