To the OP Question for me it has been
1) Too bright in inserts - they become distracting and give me a headache (esp. games like monster bash with next shot insert blinking)
2) Flicker/Ghosting/Strobing - whichever makes the insert lit when its not supposed to be (My T2 with LED's has 2x 4x and all the inlanes lit when you start the game.. huh?)
3) Brightness tends to wash out plastics instead of enhance.
4) LED's (the many i have tried) dont "wash" the playfield with light, tend to be too focused including the frosted ones i have used.
If they truly created a 1 LED to replace incandescent bulbs i would be all over it. For now, i just use them for spot areas to enhance the lighting.