it is DI dependent, and it can be SDI'd out of, but the combo potential it has is amazing. assuming you do connect the nair, you have a couple options.
if they dont DI, you can follow up with nair/uair at mid-low percents to continue the combo.
if they DI away, just knee or regrab.
its just a matter of understanding which aerials are best and reacting accordingly to your opponents DI.
a lot of falcon mains who ive talked to say downthrow>nair sucks because of smash DI, and its DI dependent, but most people arent looking out for a nair after a downthrow. they usually expect a knee. for example: at low percents downthrow>knee shouldnt work on marth til about 80% (i may be wrong, i dont pay too much attention to percents), at low percents marths dont DI away a downthrow. they DI up, or not at all to bait a knee and fair out. nair WILL connect before they can fair and can lead into a huge punish if not a 0-death combo. situations like those are why i value downthrow>nair so much.