The hardest thing I tried so far is starting in one of the poorer northern counties of Russia/Novgorod. Beloozero area and surroundings. The population is not orthodox either. The Russian princedoms are so big and dispersed that it is very difficult to grab one, they're like kingdoms in their own right. I have not succeeded yet.
I guess once you're a prince, it's smooth sailing to the Russian Kingdom.
Maybe just one game played in particular area with normal/normal settings isn't much for drawing reliable conclusions, but so far I wouldn't say that Russians (in their environment) are something specially difficult to manage.
I'm playing currently one of the Russian dynasties (famous "of Yaroslavl"-s, surely you remember them from history course

). Starting as a poor one-province count in 1066 (and a vassal of Prince of Pereyaslavl), the main goal of this game for me has been to beat mighty Rurikovichs in a DynasticGlory score sense. Just a moment ago I saved a game at year 1200, and noticed, after running the scoring application, that I have actually managed to reach my goal.

I'm still surprised it happened so quickly and easily, though (the more so, because the Rurikovichs haven't been treated by "of Yaroslavl"-s without respect (their wings are clipped, but they aren't crushed - I let them still run a second largest Russian Principality even (after all, they are my only reliable source of Russian Orthodox brides))).
Some figures:
o In 1066, Rurikovich dynasty had 53 members (32 still living at that time), and Yaroslavls just 2 members (both 2 living). In 1200, respective figures were 907 (308), and 839 (476). As a note, the Russians seem to be quite fertile, and cheerful people, by the way.
o In 1066, Rurikovichs had a total Glory 17977, and Yaroslavls 198. In 1200, respectively 228491, and 230639.
I haven't used any assassinations, or strategical (inheritance-centered) marriages, nearly all the brides have been local (Russian) girls - just a brute force and persuasion into vassalship.