I don´t see any real safety issues on that route (except maybe traffic can be a bit hairy in some places, esp. Russia and Poland.)
BTW, are you required a visa to enter Russia? They´ve managed to make their country not-so-appealing at least for Finnish people, who are just passing by, and would like to continue to Eastern Europe, because even a single-entry visa for Russia applied through a travel agency is in the region of 70-80€ per person. Nowadays, when you can take a ferry from Helsinki to Tallinn Estonia for much less, and then continue straight down towards Poland, most travellers leave Russia out completely (so the Russians don´t get a penny from them – but I´m not sure they´ve thought about it that way, it´s probable that they like their bureaucracy more than they like a few tourist dollars/euros!)
Also taking a rental bike abroad, especially to take it to Russia, could be a bit of a problem (even though they´ve joined the Green Card system, that might help a bit), but you´ll need to check that with whoever you are renting from.