In general terms;
Japanese are very rule-abiding and conservative players so I find them easy to strong-arm.
Germans are so-so. Some play decent but there are more fish than decent players.
Russians are a lottery ticket, you either get the aggressive, all-out types who don't care or those who play a patient and intelligent game.
Ukrainians stand out to me as some of the best overall - they play a very good game.
That kind of prejudice can lead you to lose many chips. It is better to observe the players and make deductions based on their movements instead of their flag.