Buying a new generation Nintendo hardware day one these days is the textbook definition of insanity, but good luck anyway OP.
Why exactly? There are legitimate reasons why it would be a bad idea for some people, but it isn't applicable to everyone.
The only actual reasons I can think of are the possibility of a future SKU that would appeal more to a specific demographic (3DSXL for those that want a larger screen) but at that point it's the same thing as waiting for a PS Slim.
It would also be silly to buy this day one if either the launch window (because we aren't going to groundhog day March 3, time will move on as much as I'm sure you wish it wouldn't) games aren't to your liking, or you simply aren't that into Nintendo first party titles.
Given that even most Nintendo detractors are simply detractors because they want Nintendo games on their console of choice rather than buying Nintendo hardware, the latter case is not likely. That leaves the launch window, which actually has a lot of good stuff but, eh, not everyone likes the same stuff, which is what it is.
It really can't be because "oh it might fail" either. Even their worst performing console of all time had some of their best first party experiences ever. I bought a Wii U day one, and I'm glad I did. The droughts sucked, I wish they got better support, but I played the hell out of some MarioKart, Yoshi, Bayonetta, Donkey Kong, etc etc.
I guess the better question is why are you so concerned about the mental wellbeing of people hyped for a Nintendo console launch, so much so that you decided to hit the "post reply" button?