The problems are total load versus on-off sensitivity.
An ordinary incandescent bulb is a resistive load, no different than a brake applied to a wheel. Hit the binders on a train, and things get mighty hot before slowly stopping the train. Just like our brake, it gets hot and can stay lit momentarily, even as the generator is on the low end of its output. That resistance helps a bulb smooth out lumpy low speed generator power. But yes, run them slow enough, even old car headlights dimmed at idle.
An LED is a different kind of light. It comes on and goes out instantaneously, with very little resistance, heat, or delay. As our generator drops to minimum speed, it effectively flutters the LED as supplied voltage wavers around the minimum required by the circuit.
Selecting usable LEDs is another matter. Just as with bulbs, ratings matter. Those designed to run at very low voltage may light bunches of lights, but may pop at high voltage if the circuit is overdriven. I’ve popped LEDs using a battery pack to check a string. The LED furthest away popped like a shotgun primer, startled me. Lol. Incandescent bulbs will blow, but give warning as they get very bright when overdriven.
The key when designing a circuit is either matching output to load, a tricky proposition when generator speed and output varies a lot, (and why we use alternators in cars nowadays), or adding a regulator. I get bogged down with circuit math, but in essence, you have to match load to output. Either govern the generator speed or use a regulator of some kind. I’d google the circuit diagram and parts list for a simple low power battery charging circuit with a regulator. Otherwise, youll pop a string of LEDs like fireworks while trying to entertain folks with your clever steamer, or you’ll cook the wiring trying to drive too many bulbs.
There are tons of different LEDs and bulbs out there. Thankfully there are circuit diagrams as well. I used to once know the numbers, but not these days. I learned after I used an unregulated output from an (AC STATOR) outboard motor to run lights. Worked great until I opened the throttle and popped the bulbs, leaving us out on the water, at night, in total darkness.