As Theodoro, you take all the alliances you can get (Trebizond, Circassia, Georgia), ally Byzantium, then roll face against Genoa and you're golden unless Crimea ganks you. It's a quick restart if they do though.
In my Theodoro game, I beat Crimea when they attacked me, due to aforementioned alliances. Crimea really isn't that strong.
I'd say Yaroslavl is the hardest in Europe and also in the game at large. I recently posted a thread asking for input on it, and it got zero responses. Guess people don't even play Yaroslavl. Its only competition is Albania and Perm, but Albania is at least independent and has weak neighbours it can annex, and Perm at the very least borders Kazan and unclaimed Siberia. Yaroslavl? Well, you're a vassal of Muscovy, and you only border Muscovy and Tver. If you somehow manage to win your independence before Tver is gone, you can annex them and border Novgorod. So what's the problem, you may ask? Well, with states like Liège, you can at least DoW Serbia or somesuch and run away. Yaroslavl has no such saving grace — you can't get access through Muscovy. You have to tear your way through them from the inside, and may only declare yourself successful on top of its bloody, beaten corpse.
Why, you may ask, do I consider Yaroslavl and Perm way worse off than French minors? Easy: you can't get
anyone to support your independence against Muscovy (except Lithuania, which gets PU'd more than 90% of the time). You're on your own, whereas if you play Foix, you can dogpile France with any combination of Castile and Austria and Burgundy and Aragon on your side.
edit: all of that said, Yaroslavl
is possible, and very satisfying as well when you succeed. Their ideas are pretty bad, but I suggest defensive + offensive + innovative to make the best out of them. Float 75 tradition and get top-tier generals for the rest of the game.