Actually when I started getting acquainted with the behind the scenes work in the last few BBC nature documentaries, I got very disenchanted with the whole nature film-making process.
However, in recent years I've done some amateur videography of amphibians in the wild (nothing public yet) and it really is surprisingly complicated to film in wild conditions. Much more difficult than photographing amphibians in the wild.
Lighting is a huge challenge. Sound has to be recorded separately in most cases and it needs a lot of processing. Film sequences have to be set-up in advance, and choreographed. And then you have to get the animals to be in the right place, at the right time, doing what you want them to do.
Many behaviors, including dart frogs feeding tadpoles, are so much more accessible if filmed in a terrarium under controlled conditions. Not to mention the fact that the BBC didn't have to send a crew to Peru just to film that sequence.
The only aspect of these films that bugs me still is the fact that the audio is by-and-large completely artificial (i.e. it wasn't recorded at the time the footage was taken).