Until recently, the Cubs have not had much luck with signing players from Nippon Pro Baseball (NPB) in Japan.
Kosuke Fukudome was (mostly) a bust. Kyuji Fujikawa and Koji Uehara, forgettable relievers. The Cubs did get some good performance from Yu Darvish, but he was a known quantity in MLB before he came to the Cubs.
In general, other than Darvish, Japanese pitchers have not fared well in MLB. Hideo Nomo did, for a time. Daisuke Matsuzaka was supposed to be dominant, but after his first two years he wasn’t very good.
So why would Yoshinobu Yamamoto be any different?