I'll throw my 2 cents in since I have one.
So heres the deal with the NH-D15, its heavy as hell. While it does mount through the motherboard and has a back plate, it still requires your motherboard to support it. Depending on the motherboard you may have a thick PCB or a thin PCB (I'm looking at you Asrock) and thin ones will have more of a threat of breaking than the thicker boards. But thats it is, a threat, not a sure thing. Now where the worry comes in is mostly for transporting the thing, like shipping it or driving to a LAN with it kind of transport. This to an extent goes for all huge tower coolers, but the NH-D15 being one of the heaviest gets the most attention. Some reviewers online have gotten broken motherboard because of this cooler (and other huge air coolers) but thats because of shipping issues, not because the cooler will inherently break your board. If you're just using it in your computer at home, and the only time you move it is to clean it out, then you really have nothing to worry. Just don't jerk it sharply around and it'll be fine.
All that being said, unless you plan on doing a lot of OCing, the thing is overkill for damn near every situation. It also makes RAM selection to be something of a pain in the ass. If you just want a big quiet air cooler, I'd recommend looking at the Cryorig H7 or for bigger H5 Universal. What's your build and what do you plan on doing with it?