Is frequency of 40 Hz light from Apple/Android tablet as reliable as that from an LED light?
Its probably not because of the OS getting in the way (although they always get in the way when timing is important). 40Hz is so long compared to the processor clock speed I would think it would be fine. At 1GHz you could be off by 100,000 cycles and still be within 0.4%.of 40Hz.
Probably a bigger factor is the 60Hz-120Hz LCD refresh. 40Hz is a multiple of 120Hz so that might be okay, but other common  refresh rates, not so much.