? barely, if at all:
- Byakugan cannot counter Susano'o / PS
- Byakugan cannot counter Genjutsu / Tsukuyomi / Kotoamatsukami
- Byakugan cannot counter Amaterasu / Enton
- Byakugan cannot counter Kamui's absorption or intangibility
- Byakugan cannot counter Sharingan's precog (as in, it wouldn't negate or cancel the precog ability, but due to Byakugan the fight would be more even)
What the heck can it actually counter, let alone, "very hard counter"? All that the Byakugan does is almost 360º field of vision, farsight and see chakra network activity. At the very best, it would sense the chakra building in the eyes for Amaterasu (like Nagato sensed) but for that, you would also need to know how amaterasu works in the first place, or else it would be useless.
P.S: No, Kaiten is not defending against Susano, Amaterasu or anything else.