I don't shoot video but if you search online you would have more info and insight about A9ii than can be offered here. That being said I will put myself in your situation and answer your question.
1. Is A9ii meant for you, in other words are you their (sony) target customer? If you are sony fanboy or A7 user and have spare cash then by all means enjoy it.
2. What is primarily A9ii designed for? Answer: A sports action stills camera. Do note 1DX3 is a dslr (mirror) and A9ii mirrorless camera.
3. Video performance, 1DX3 or A9ii better? Well it's relative, your question is A9ii has IS specifically IBIS (in body image stabalisation) where there is none on 1DX3. It's a useful feature to have as videographers like to use adapted manual lenses.
3a. 1DX3 video specs is superior than A9ii but in general unless you are a video pro you would not use A9ii or 1DX3 either. In otherwords A9ii is very usable for enthusiasts.
3b. Autofocus during video shooting. In past sony models like eye autofocus only work for stills photography perhaps also face recognition but I could be wrong. So I think all this is brought to A9ii for video as the processor is more capable. More importantly the ability to track autofocus while subject is moving towards camera. Below is link of how well both cameras track autofocus in shooting stills mode.
In my own opinion, if you can afford A9ii it's your money to spend although there is the A7 series for video specifically the A7S series meant for video use but features and performance vary.
