He's not coming out of nowhere at all... he has been on many Klitschko undercards in the last couple of years, and has beaten a few reasonable names. I've seen him fight many times, and the discription boxing fanatic gave you is pretty accurate.
Ustinov is a former kick boxer, tough as nails, never been hurt from a punch or a kick in his boxing and kickboxing days.
I assumed Ustinov was **** when he struggled and went the distance against Monte Barrett but then Barrett did his thing against Tua twice and Ustinov has looked marginally better against other guys so Ustinov has risen slightly in my estimation. Pulev will completely outclass him though.
All I'm saying is Ustinov is slow as hell and has no idea how to box. He's strong and tough, but tends to wear guys down by throwing his body around a lot rather than actually landing good punches. Even a mediocre defensive fighter like Pulev should have no problem avoiding his molasses-like punches.