I finished my first documentary this year and made plenty of mistakes. There are so many elements to video and cinematography that aren't being address by photographers.
Most professional cinematographers don't own their own cameras, because they are too expensive (but that is changing) and because no one wants the same format, maybe Geo wants 1080P with a Panasonic while PPS wants 720P with a Sony and BBC want 4K RAW only. The chances of you having the camera that they want to use is almost zero.
Of course if you are doing your own projects and plan to do all your own editing, audio and final output, then it doesn't really matter what camera you use.
HD stock agencies don't want anything a DSLR outputs, because the bit-rate is too low, the bare minimum is 35mps.
The safest all-around camera that won't completely break you is the Sony EX1-EX3. You can get an adapter that allows you to use your Canon or Nikon glass. They are the only cameras under 8,000 that are approved for broadcast by the BBC and Discovery.
Personally, without true variable frame rates, I find the DSLR video cameras pretty useless for nature photography.