Expectations for using DSLR to take video

[im1dermike]

I purchased a Nikon D3400 within the past year. Before purchasing, I was thinking that a "nice to have" feature would be the ability to take video (which, of course, the D3400 does, albeit by draining battery quickly). Ideally, I would be able to replace my Canon VIXIA HF52 (cheap consumer camcorder) with the D3400. Besides only having to carry around a single piece of equipment if it all worked out, I've always had issues with graininess with the Canon camcorder in any situation without a lot of light (which is usually the case).

I've only tried taking video with the D3400 a few times, but, while the picture quality is excellence, I ran into issue focusing. I'm not sure if this is entirely caused by the fact that I almost always have a 35mm prime lens on or the mode I was shooting in or what.

Should I expect to be able to use my D3400 as a camcorder or are the DSLRs inherently "not the best choice" for video? If the former, what lens/mode would be best?