For a neck, I agree that it's not stiff enough to be a good choice.
There's a lot of voodoo and tradition flying around out there about what's suitable to use and what's not. A LOT of it revolves around what the companies use to make an electric, and have been using for decades. They're all about production efficiency and cost effectiveness. They don't use a rosewood for anything but fretboards not because it doesn't sound good, but because it'd be too dang expensive.
With that in mind, consider the comparitive cost of poplar to maple. Maple is more expensive, yet they choose to use it anyway. THAT'S the sort of "voodoo" you should take serious note of.
So ask yourself why they wouldn't use a particular wood for a function. That will probably tell you if it's suitable or not.
What you look for in a neck wood is 1) strength to withstand the tension of the strings, 2) stability (no/little movement, twisting, warping), and 3) tone balancing against the tone of the body wood. So long as #s 1 & 2 are met, the rest is up to you.
For a body, you want it to be able to be shaped well and to be able to hold the screws tightly. Low weight is a plus.