DSLRs seem to be the tools of the trade because the price is right & they can look great if you invest in nice glass. personally i feel limited by them without proper histograms etc. but if you aren’t concerned with those things then a DSLR should work fine.

BUT sound is obviously a very important factor for documentaries and DSLRs definitely don’t record good in-camera audio, so if you buy a DSLR it’s wise to invest in an external Zoom recorder