The main problem with soviet/russian/ukrainian steel is maintenance. which skyrockets quickly. Usually, one has to get rid off the birds before their first D Check.
But their very low acquisition costs can make them excellent "growth" aircraft. After some growth, though, you'll want to get rid of them and go to better western aircraft. (NB : the SSJ is intermediate in that domain, half way between both. IRL it is sold as a growth plane to replace later with A320s)