GI is often AC. Depending on the LEDs and whether the power is rectified with diode bridges or not, only half the wave will light the LEDs. At the low frequency used, this strobing is easy for the eye to see. This is especially noticeable as the eye scans the field, leaving a momentary dashed pattern on your retina from them flicker and eye movement. If rectified, fewer people will see it at full brightness. Keeping power saving off becomes essential.
There are ways to smooth this out but it adds to the parts and assembly cost. Rectifier bridges to see the full AC power wave instead of just the half; adding a capacitor to charge/discharge to further even the voltage out before reaching he LEDs.
Another option is Herg's LED OCD and GI OCD boards. They are still AC but much much higher frequency. The B/W GI boards are programmable on how much of a duty cycle to use on the LEDs as well as how quickly to ramp up/down the brightness when the game applies or removes the signal. This way incandescent can be emulated. Herg's Stern GI board is simple dumb and cheap - just basically a regulated and smoothed DC supply. All of these are really only options if you want to led the entire game; and work best with non specialized LEDs. Don't need anti flicker or anti ghosting LEDs; simply get the cheapest LEDs that give you the color and brightness you desire.
Coming back to your string of lights: